Cygwin: Makefile: only regenerate cygwin_version.c if it changes

Cygwin: Makefile: only regenerate cygwin_version.c if it changes

Make sure to create a new cygwin_version.c if it either doesn't
exist yet, or if it would be different from the former file.
This avoids unnecessary DLL rebuilding.

Fixes: 97eb64b909bc ("Cygwin: uname: generate default release string from git as well")
Fixes: 4949a82cde98 ("Cygwin: uname: fix building in non-git source dir")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>