download_and_compile.sh: pass -DTRANSLATIONS_SRC_DIR when building FlightGear

When configuring FlightGear, flightgear/CMakeModules/Translations.cmake
checks for the presence of "${FGDATA_SRC_DIR}/Translations/${xlf_file}"
where ${xlf_file} is en_US/FlightGear-Qt.xlf, however this
${FGDATA_SRC_DIR} is *not* initialized from -DFG_DATA_DIR argument
passed on FlightGear's cmake command. The autodetection only works if
FGData is located in ../fgdata from the FlightGear source directory. In
the meantime, make download_and_compile.sh explicitly pass
-DTRANSLATIONS_SRC_DIR:PATH when configuring FG.

The -DFG_DATA_DIR:PATH vs. -DFG_DATA_DIR change is visible in
build/flightgear/CMakeCache.txt but not decisive here.

download_and_compile.sh

@@ -927,7 +927,8 @@ if _elementIn "FGFS" "${WHATTOBUILD[@]}" || \
             -DENABLE_FLITE=ON \
             -DCMAKE_INSTALL_PREFIX:PATH="$INSTALL_DIR_FGFS" \
             -DCMAKE_PREFIX_PATH="$INSTALL_DIR_SIMGEAR;$INSTALL_DIR_OSG;$INSTALL_DIR_OPENRTI;$INSTALL_DIR_PLIB" \
-            -DFG_DATA_DIR="$INSTALL_DIR_FGFS/fgdata" \
+            -DFG_DATA_DIR:PATH="$INSTALL_DIR_FGFS/fgdata" \
+            -DTRANSLATIONS_SRC_DIR:PATH="$INSTALL_DIR_FGFS/fgdata/Translations" \
             $FG_CMAKEARGS \
             ../../flightgear 2>&1 | _logOutput
     fi