build: Expand version info in SDL_REVISION and SDL_GetRevision()

Instead of using a URL and git sha1, this uses `git describe` to
describe the version relative to the nearest previous git tag, which
gives a better indication of whether this is a release, a prerelease,
a slightly patched prerelease, or a long way after the last release
during active development.

This serves two purposes: it makes those APIs more informative, and it
also puts this information into the binary in a form that is easy to
screen-scrape using strings(1). For instance, if the bundled version of
SDL in a game has this, we can see at a glance what version it is.

It's also shorter than using the web address of the origin git
repository and the full git commit sha1.

Also write the computed version into a file ./VERSION in `make dist`
tarballs, so that when we build from a tarball on a system that doesn't
have git available, we still get the version details.

For the Perforce code path in showrev.sh, output the version number
followed by the Perforce revision, in a format reminiscent of
`git describe` (with p instead of g to indicate Perforce).

For the code path with no VCS available at all, put a suffix on the
version number to indicate that this is just a guess (we can't know
whether this SDL version is actually a git snapshot or has been
patched locally or similar).

Resolves: https://github.com/libsdl-org/SDL/issues/6418
Signed-off-by: Simon McVittie <smcv@collabora.com>

Makefile.in

@@ -252,7 +252,7 @@ dist $(distfile):
 	    -name '.#*' \) \
 	    -exec rm -f {} \;
 	if test -f $(distdir)/test/Makefile; then (cd $(distdir)/test && make distclean); fi
-	(cd $(distdir); build-scripts/updaterev.sh)
+	(cd $(distdir); $(srcdir)/build-scripts/updaterev.sh --dist)
 	tar cvf - $(distdir) | gzip --best >$(distfile)
 	rm -rf $(distdir)