download_and_compile.sh: improve the --help output

Reorder options (most basic and important options first; related options
together; experimental or deprecated options last). Explain a bit more
in some cases. Use the https protocol in the URL of the FlightGear wiki.

download_and_compile.sh

@@ -855,28 +855,22 @@ be one of the following words:
 Available options:
   -h, --help    show this help message and exit
       --version print version and license information, then exit
-  -e            compile FlightGear with --with-eventinput option (experimental)
-  -i            compile SimGear and FlightGear with -D ENABLE_RTI=ON option (experimental)
-  -b RELEASE_TYPE                                                                     default=RelWithDebInfo
-                set build type to RELEASE_TYPE (Release|RelWithDebInfo|Debug)
-  -a y|n        y=run 'PACKAGE_MANAGER update', n=don't                               default=y
-                (PACKAGE_MANAGER being a program like 'apt-get', see below)
-  -p y|n        y=install packages using PACKAGE_MANAGER, n=don't                     default=y
-  -c y|n        y=compile programs, n=don't                                           default=y
-  -d y|n        y=fetch programs from the Internet (Git, svn, etc.), n=don't          default=y
+  -s            compile the latest stable release of FlightGear (and
+                corresponding versions of the other components)
+      --lts     compile the latest Long Term Stable release of FlightGear (and
+                corresponding versions for other components)
+      --old-lts compile the previous Long Term Stable release of FlightGear (and
+                corresponding versions for other components)
       --cleanup Remove all build and installation directories. Try this if a
                 compilation fails and the \"base directory\" was not \"fresh\" when
                 you started ${PROGNAME}. If this option is given and no
                 component has been specified on the command line, then only the
                 cleanup routine is run.
-      --git-clone-default-proto=PROTO                                                 default=https
-                default protocol to use for 'git clone' (https, git or ssh)
-      --git-clone-site-params=SITE=PROTOCOL[:USERNAME]
-                use PROTOCOL as USERNAME when cloning a Git repository located
-                at SITE (sample sites: 'SourceForge', 'GitHub'; valid
-                protocols: 'ssh', 'https', 'git'; USERNAME is required when
-                using 'ssh'). You may pass this option several times with
-                different sites.
+  -d y|n        y=fetch programs from the Internet (Git, svn, etc.), n=don't          default=y
+  -p y|n        y=install packages using PACKAGE_MANAGER, n=don't                     default=y
+                (see --package-manager and --sudo)
+  -a y|n        y=run 'PACKAGE_MANAGER update', n=don't                               default=y
+                (PACKAGE_MANAGER being a program like 'apt-get', see below)
       --package-manager=PACKAGE_MANAGER                                               default=apt-get
                 program used to install packages; must be compatible with
                 'apt-get' for the operations performed by $PROGNAME.
@@ -886,21 +880,35 @@ Available options:
                 Passing 'echo' as the SUDO_PROGRAM can be useful to see what
                 would be done with the package manager without actually running
                 the commands.
-  -j X          pass -jX to the Make program
+  -c y|n        y=compile programs, n=don't                                           default=y
+  -r y|n        y=reconfigure programs before compiling them, n=don't                 default=y
+                (for SimGear and FlightGear, reconfiguring means running CMake)
+  -b RELEASE_TYPE                                                                     default=RelWithDebInfo
+                set build type to RELEASE_TYPE (Release|RelWithDebInfo|Debug)
+  -j X          pass -jX to the Make program (X is the number of jobs to run
+                simultaneously; for instance, if your CPU has n cores, passing n
+                or slightly less as the X value will speed up compilation a lot)
   -O X          pass -OX to the Make program
-  -r y|n        y=reconfigure programs before compiling them, n=don't reconfigure     default=y
+      --sg-cmake-arg=ARGUMENT
+                Pass ARGUMENT to CMake when builing SimGear.
+                This option may be given several times.
+      --fg-cmake-arg=ARGUMENT
+                Pass ARGUMENT to CMake when building FlightGear.
+                This option may be given several times.
       --ignore-intercomponent-deps
                 Ignore dependencies between components (default: don't).
                 Example: TERRAGEAR depends on SIMGEAR. Passing the option can be
                 useful if you want to update, rebuild, etc. TERRAGEAR without
                 doing the same for SIMGEAR (e.g., if doing repeated TERRAGEAR
                 builds and you know your SIMGEAR is already fine and up-to-date).
-      --lts     compile the latest Long Term Stable release of FlightGear (and
-                corresponding versions for other components)
-      --old-lts compile the previous Long Term Stable release of FlightGear (and
-                corresponding versions for other components)
-  -s            compile the latest stable release of FlightGear (and
-                corresponding versions of the other components)
+      --git-clone-default-proto=PROTO                                                 default=https
+                default protocol to use for 'git clone' (https, git or ssh)
+      --git-clone-site-params=SITE=PROTOCOL[:USERNAME]
+                use PROTOCOL as USERNAME when cloning a Git repository located
+                at SITE (sample sites: 'SourceForge', 'GitHub'; valid
+                protocols: 'ssh', 'https', 'git'; USERNAME is required when
+                using 'ssh'). You may pass this option several times with
+                different sites.
       --override-repo=COMPONENT=SITE:ADDRESS
                 Override the repository from which COMPONENT is fetched when
                 its repository is cloned (initial retrieval). SITE is defined
@@ -916,21 +924,17 @@ Available options:
                 Override the default branch for COMPONENT. For the specified
                 component, this overrides the effect of options -s, --lts and
                 --old-lts. This option may be given several times.
-      --sg-cmake-arg=ARGUMENT
-                Pass ARGUMENT to CMake when building SimGear.
-                This option may be given several times.
-      --fg-cmake-arg=ARGUMENT
-                Pass ARGUMENT to CMake when building FlightGear.
-                This option may be given several times.
-      --compositor
-                build FlightGear with compositor enabled
       --non-interactive
                 don't ask any question; always assume the default answer in
                 situations where a question would normally be asked.
+      --compositor
+                build FlightGear with compositor enabled
+  -e            compile FlightGear with --with-eventinput option (experimental)
+  -i            compile SimGear and FlightGear with -D ENABLE_RTI=ON option (experimental)
 
 More detailed information can be found on the FlightGear wiki:
 
-  http://wiki.flightgear.org/Scripted_Compilation_on_Linux_Debian/Ubuntu
+  https://wiki.flightgear.org/Scripted_Compilation_on_Linux_Debian/Ubuntu
 
 The wiki may sometimes be a bit outdated; if in doubt, consider this help text
 as the reference."