Adjust build scripts to set FG_BUILD_TYPE

Cmake now has a standard variable to indicate the build type we are
creating. Map the existing settings on each platform to that value
(with varying degrees of nice-ness, further clean-up possible)

build_release_windows.bat

@@ -1,6 +1,12 @@
 
 IF NOT DEFINED WORKSPACE SET WORKSPACE=%~dp0
 
+IF %IS_NIGHTLY_BUILD% EQU 1 (
+  SET FGBUILDTYPE=Nightly
+) ELSE (
+  SET FGBUILDTYPE=Release
+)
+
 REM following are for testing the script locally
 REM SET PATH=%PATH%;%ProgramFiles%\CMake\bin;%ProgramFiles(x86)%\"Inno Setup 5"\
 REM SET QT5SDK32=C:\Qt\5.6\msvc2015
@@ -27,7 +33,8 @@ cmake ..\flightgear -G "Visual Studio 14" ^
                     -DCMAKE_INSTALL_PREFIX:PATH=%WORKSPACE%/install/msvc140 ^
                     -DCMAKE_PREFIX_PATH:PATH=%WORKSPACE%/install/msvc140/OpenSceneGraph ^
                     -DBOOST_ROOT=%WORKSPACE%/windows-3rd-party ^
-                    -DCMAKE_PREFIX_PATH=%QT5SDK32%;%OSG32%
+                    -DCMAKE_PREFIX_PATH=%QT5SDK32%;%OSG32% ^
+                    -DFG_BUILD_TYPE=%FGBUILDTYPE%
 cmake --build . --config RelWithDebInfo --target INSTALL
 
 cd ..
@@ -49,7 +56,8 @@ cmake ..\flightgear -G "Visual Studio 14 Win64" ^
                     -DMSVC_3RDPARTY_ROOT=%WORKSPACE%/windows-3rd-party/msvc140 ^
                     -DBOOST_ROOT=%WORKSPACE%/windows-3rd-party ^
                     -DCMAKE_INSTALL_PREFIX:PATH=%WORKSPACE%/install/msvc140-64 ^
-                    -DCMAKE_PREFIX_PATH=%QT5SDK64%;%OSG64%
+                    -DCMAKE_PREFIX_PATH=%QT5SDK64%;%OSG64% ^
+                    -DFG_BUILD_TYPE=%FGBUILDTYPE%
 cmake --build . --config RelWithDebInfo --target INSTALL
 
 cd ..