- When FGDATA_SRC_DIR isn't explicitly set by the user, set it to
${PROJECT_SOURCE_DIR}/../fgdata if this is a directory.
- When TRANSLATIONS_SRC_DIR isn't explicitly set by the user, look for
translations in ${FGDATA_SRC_DIR}/Translations instead of
${PROJECT_SOURCE_DIR}/../fgdata/Translations (i.e., honor the setting
of FGDATA_SRC_DIR for the sake of translation lookup).
This fixes the problem explained in [1] and [2].
[1] https://sourceforge.net/p/flightgear/mailman/message/36645499/
[2] b2274fbbed/