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 -DFUZZEL_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 +1,38 @@
+#!/bin/sh
+
+set -e
+
+default_version=${1}
+src_dir=${2}
+out_file=${3}
+
+# echo "default version:  ${default_version}"
+# echo "source directory: ${src_dir}"
+# echo "output file:      ${out_file}"
+
+if command -v git > /dev/null; then
+    workdir=$(pwd)
+    cd "${src_dir}"
+    git_version=$(git describe --always --tags)
+    git_branch=$(git rev-parse --abbrev-ref HEAD)
+    cd "${workdir}"
+
+    new_version="${git_version} ($(env LC_TIME=C date "+%b %d %Y"), branch '${git_branch}')"
+else
+    new_version="${default_version}"
+fi
+
+new_version="#define FOOT_VERSION \"${new_version}\""
+
+if [ -f "${out_file}" ]; then
+    old_version=$(cat "${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