Correctly generate FISH_BUILD_VERSION for use in fish_version.h/cpp and
fish.pc to allow `fish --version` and `echo $version` to work again.
Not needing the same convoluted measures used by Makefile builds to
prevent the regeneration of the fish version file when it hasn't
changed.
Purposely created a new `cmake_git_version_gen.sh` file so that the old
`git_version_gen.sh` remains compatible with the existing Makefile build
script. Same reason why `fish.pc.in` was not modified to use a lowercase
variable name to match the CMAKE variable of the same name.
Closes#4626
If `doxygen` isn't installed, the man files aren't built and that's
quite ok. The cmake `install` target was presuming the man files would
always be present and the install stage was failing if they weren't
built.
