b2274fbbed
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.