meson: allow setting the version string during configuration

This will be useful when building distro packages, because we can set the
version string to the rpm/dpkg/whatever version string, and getter reports
from end users.

meson.build

@@ -585,6 +585,7 @@ endif
 
 vcs_tagger = [meson.source_root() + '/tools/meson-vcs-tag.sh',
               meson.source_root(),
+              get_option('version-tag'),
               meson.project_version()]
 
 sed = find_program('sed')

meson_options.txt

@@ -1,6 +1,9 @@
 # -*- mode: meson -*-
 # SPDX-License-Identifier: LGPL-2.1+
 
+option('version-tag', type : 'string',
+       description : 'override the git version string')
+
 option('split-usr', type : 'combo', choices : ['auto', 'true', 'false'],
        description : '''/bin, /sbin aren't symlinks into /usr''')
 option('split-bin', type : 'combo', choices : ['auto', 'true', 'false'],

tools/meson-vcs-tag.sh

@@ -4,7 +4,13 @@ set -eu
 set -o pipefail
 
 dir="$1"
-fallback="$2"
+tag="$2"
+fallback="$3"
+
+if [ -n "$tag" ]; then
+        echo "$tag"
+        exit 0
+fi
 
 # Apparently git describe has a bug where it always considers the work-tree
 # dirty when invoked with --git-dir (even though 'git status' is happy). Work