Teach Xcode to run new script xcode_version_gen.sh before building the fish_shell and fish_indent targets. The script generates file fish-build-version.h for inclusion by fish_version.cpp. Note that Xcode always runs the script because of the phony target named force-fish-build-version.h, but fish-build-version.h is only touched if the contents of FISH-BUILD-VERSION-FILE change. Fixes #890