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
