download_and_compile.sh: update TerraGear dependencies and branch selection

- update dependencies of the TERRAGEAR component (require SimGear,
  libboost-dev and zlib1g-dev apart from what was already listed
  following [0]);

- use TerraGear branch 'scenery/ws2.0' if the -s option has been given,
  otherwise use the 'next' branch.

References: [1] and Scott's reply[2].

[0] https://sourceforge.net/p/flightgear/fgmeta/ci/6661d1e10cae398154ada920551fdb7237ec035b/
[1] https://sourceforge.net/p/flightgear/mailman/message/36649774/
[2] https://sourceforge.net/p/flightgear/mailman/message/36650028/

download_and_compile.sh

@@ -615,7 +615,11 @@ if [[ "$DOWNLOAD_PACKAGES" = "y" ]]; then
 
   # TerraGear
   if _elementIn "TERRAGEAR" "${WHATTOBUILD[@]}"; then
-    PKG+=(libcgal-dev libgdal-dev libtiff5-dev)
+    # Require SimGear
+    if ! _elementIn "SIMGEAR" "${WHATTOBUILD[@]}"; then
+      WHATTOBUILD+=(SIMGEAR)
+    fi
+    PKG+=(libboost-dev libcgal-dev libgdal-dev libtiff5-dev zlib1g-dev)
   fi
 
   # TerraGear GUI and OpenRTI
@@ -1183,7 +1187,14 @@ if _elementIn "TERRAGEAR" "${WHATTOBUILD[@]}"; then
   mkdir -p "terragear"
   cd "$CBD"/terragear
   _gitDownload TERRAGEAR
-  _gitUpdate scenery/ws2.0
+
+  if [ "$STABLE" = "STABLE" ]; then
+    branch='scenery/ws2.0'
+  else
+    branch='next'
+  fi
+
+  _gitUpdate "$branch"
 
   if [ "$RECONFIGURE" = "y" ]; then
     cd "$CBD"