build: Add a mechanism to mark builds with vendor info

Downstream distributors can use this to mark a version with their
preferred version information, like a Linux distribution package version
or the Steam revision it was built to be bundled into, or just to mark
it with the vendor it was built by or the environment it's intended to
be used in.

For instance, in Debian I'd use this by configuring with:

    --enable-vendor-info="${DEB_VENDOR} ${DEB_VERSION}"

to get a SDL_REVISION like:

    release-2.24.1-0-ga1d1946dc (Debian 2.24.1+dfsg-2)

which gives a Debian user enough information to track down the patches
and build-time configuration that were used for package revision 2.

In Autotools and CMake, this is a configure-time option like any other,
and will go into both SDL_REVISION (via SDL_revision.h) and
SDL_GetRevision().

In other build systems (MSVC, Xcode, etc.), defining the
SDL_VENDOR_INFO macro will get it into the output of SDL_GetRevision(),
although not SDL_REVISION.

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

include/SDL_revision.h

@@ -1,2 +1,6 @@
+#ifdef SDL_VENDOR_INFO
+#define SDL_REVISION SDL_VENDOR_INFO
+#else
 #define SDL_REVISION ""
+#endif
 #define SDL_REVISION_NUMBER 0