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>

build-scripts/updaterev.sh

@@ -6,10 +6,28 @@ outdir=`pwd`
 cd `dirname $0`
 srcdir=..
 header=$outdir/include/SDL_revision.h
+dist=
+
+while [ "$#" -gt 0 ]; do
+    case "$1" in
+        (--dist)
+            dist=yes
+            shift
+            ;;
+        (*)
+            echo "$0: Unknown option: $1" >&2
+            exit 2
+            ;;
+    esac
+done
 
 rev=`sh showrev.sh 2>/dev/null`
 if [ "$rev" != "" ]; then
-    echo "#define SDL_REVISION \"$rev\"" >"$header.new"
+    if [ -n "$dist" ]; then
+        echo "$rev" > "$outdir/VERSION"
+    fi
+    echo "/* Generated by updaterev.sh, do not edit */" >"$header.new"
+    echo "#define SDL_REVISION \"SDL-$rev\"" >>"$header.new"
     echo "#define SDL_REVISION_NUMBER 0" >>"$header.new"
     if diff $header $header.new >/dev/null 2>&1; then
         rm "$header.new"