Remove duplication in version info target creation

Now the custom target created for the git version info generation uses
gmx_add_custom_output_target(), removing comments etc. that were
duplicated in both places and clarifying the code.  The custom target
created with gmx_configure_version_file() now also integrates with the
dependency resolution mechanism used by gmx_add_custom_output_target().

Change-Id: I868264498c014041f551df8819fb2b568d85bc30