meson: fix version generation from git

run_command() was only run at configure time, meaning the generated
version (that was passed on to the sources via -DYAMBAR_VERSION)
became stale.

Fix by implementing a shell script that generates a header file, and
wrap this in a custom target that is run every time (but the generated
file is only updated when the version changes)

generate-version.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/bash
+
+set -e
+
+default_version=${1}
+src_dir=${2}
+out_file=${3}
+
+# echo "default version:  ${default_version}"
+# echo "source directory: ${src_dir}"
+# echo "output file:      ${out_file}"
+
+if [[ $(command -v git) ]]; then
+    new_version="$(git describe --always --tags) ($(env LC_TIME=C date "+%b %d %Y"), branch '$(git rev-parse --abbrev-ref HEAD)')"
+else
+    new_version="${default_version}"
+fi
+
+new_version="#define YAMBAR_VERSION \"${new_version}\""
+
+if [[ -f "${out_file}" ]]; then
+    old_version=$(<"${out_file}")
+else
+    old_version=""
+fi
+
+# echo "old version: ${old_version}"
+# echo "new version: ${new_version}"
+
+if [[ "${old_version}" != "${new_version}" ]]; then
+    echo "${new_version}" > "${out_file}"
+fi