download_and_compile.sh: respect the selected build type for TerraGear

So far, TerraGear was always built by d&c with
-DCMAKE_BUILD_TYPE="Debug". Following [1], d&c will now use, for
TerraGear, the build type selected with its -b option---which by default
is 'RelWithDebInfo'.

(In short: the 'Debug' build type has a very high negative performance
impact, therefore it should only be done intentionally.)

[1] https://sourceforge.net/p/flightgear/mailman/message/36650766/

download_and_compile.sh

@@ -1265,7 +1265,7 @@ if _elementIn "TERRAGEAR" "${WHATTOBUILD[@]}"; then
     mkdir -p build/terragear
     cd "$CBD"/build/terragear
     rm -f CMakeCache.txt
-    "$CMAKE" -DCMAKE_BUILD_TYPE="Debug" \
+    "$CMAKE" -DCMAKE_BUILD_TYPE="$BUILD_TYPE" \
           -DCMAKE_INSTALL_PREFIX:PATH="$INSTALL_DIR_TG" \
           -DCMAKE_PREFIX_PATH="$INSTALL_DIR_SIMGEAR;$INSTALL_DIR_CGAL" \
           ../../terragear/ 2>&1 | _logOutput