build: Revamp git versioning.

build: Revamp git versioning.

This rewrites GIT-VERSION-GEN and renames it to version-gen.sh. The
main difference is that the script now generates version.c rather
than version.h, speeding up the build because switching branches
or transitioning from a clean to a dirty work-tree (or vice versa)
changes the version string. Before the patch, all files which included
version.h were recompiled. With the patch applied, only the short
version.c needs to be recompiled.

The generated version,c defines one function which returns the
single-line version string and a second function which returns
the multi-line version information (author, copyright, git URL
etc). This information is now passed to version-gen.sh via environment
variables. The CPP macros we had before are removed.