Patch is suggested by Adrian Bunk, to prevent unexpected
reflection of environmental LIBTOOL. See:
http://savannah.nongnu.org/patch/?7290
* builds/unix/unix-cc.in: LIBTOOL is unconditionally set to
$(FT_LIBTOOL_DIR)/libtool. FT_LIBTOOL_DIR is set to $(BUILD_DIR)
by default.
* configure: When configured for the building out of source tee,
FT_LIBTOOL_DIR is set to $(OBJ_DIR).