gen-demo-header.py: Fix running without git

The script assumes that the git program is in the PATH which may not be always
the case, so if git was not found, fallback to the former behavior of making
VCS_TAG an empty string if the profile option was not set to 'devel'.

Also fix the call to the script so that we do indeed call GIT on the source GIT
repo so that we can get the right short SHA1 sum.

Re-organize the code a bit in the process, so that things seem cleaner.

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

@@ -4,7 +4,8 @@ import os
 import subprocess
 import sys
 
-profile = sys.argv[1]
+repodir = sys.argv[1]
+profile = sys.argv[2]
 
 sys.stdout.write("/* This file is auto-generated. Do not edit. */\n")
 sys.stdout.write("#pragma once\n")
@@ -12,10 +13,14 @@ 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")
+if short_sha is None:
+    cmd = ["git", "-C", repodir, "rev-parse", "--short", "HEAD"]
+    try:
+        with subprocess.Popen(cmd, stdout=subprocess.PIPE) as p:
+            short_sha = p.stdout.read().decode('utf-8').rstrip("\n")
+    except FileNotFoundError:
+        short_sha = ''
+        if profile != 'default':
+            short_sha = 'devel'
+
+sys.stdout.write(f"#define VCS_TAG \"{short_sha}\"\n")

demos/meson.build

@@ -3,7 +3,7 @@ demo_profile = get_option('profile')
 
 demo_conf_h = declare_dependency(
   sources: custom_target('demo-header',
-    command: [gen_demo_header, demo_profile],
+    command: [gen_demo_header, meson.source_root(), demo_profile],
     capture: true,
     output: 'demo_conf.h',
     build_by_default: true,