build: Check CI envvars before rev-parse

The CI pipeline does not do a full clone, so we cannot use `git
rev-parse` to get the short SHA checksum for the current commit.

Of course, since nothing's ever easy, we cannot use vcs_tag() with a
custom command, so we need to generate our out `demo_conf.h` header
straight from a script.

build-aux/meson/gen-demo-header.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+
+import os
+import subprocess
+import sys
+
+profile = sys.argv[1]
+
+sys.stdout.write("/* This file is auto-generated. Do not edit. */\n")
+sys.stdout.write("#pragma once\n")
+sys.stdout.write("\n")
+sys.stdout.write(f"#define PROFILE \"{profile}\"\n")
+
+short_sha = os.environ.get('CI_COMMIT_SHORT_SHA')
+if short_sha is not None:
+    sys.stdout.write(f"#define VCS_TAG \"{short_sha}\"\n")
+else:
+    cmd = ["git", "rev-parse", "--short", "HEAD"]
+    with subprocess.Popen(cmd, stdout=subprocess.PIPE) as p:
+        short_sha = p.stdout.read().decode('utf-8').rstrip("\n")
+        sys.stdout.write(f"#define VCS_TAG \"{short_sha}\"\n")