meson: fix warning about unexpected return code checking for run_command

Recent versions of Meson warn you that the default is to not check the
return code, which is a bad default and may eventually change. To
suppress this warning, an explicit `check: ` value must be set.

Considering the code in play here:
- check if git exists
- if so, always assume this is running from a git checkout
- embed either '()' or '(git describe version)'

it seems likely the intention is indeed to have it be `check: false`,
but there's some missing error checking here to ensure it.

Check the returncode. If git fails, it is surely because there is no git
repository and the build is being run from a tarball. In that case,
behave as though git wasn't found in the first place, and use the
fallback value.

src/meson.build

@@ -20,13 +20,14 @@ config_file = configure_file(
   output : 'config.h',
   configuration : config_cfg)
 
+intel_driver_git_version = intel_vaapi_driver_version
 if git.found()
   git_version = run_command(
     git, '--git-dir', join_paths(meson.source_root(), '.git'),
-    'describe', '--tags')
-  intel_driver_git_version = git_version.stdout().strip()
-else
-  intel_driver_git_version = intel_vaapi_driver_version
+    'describe', '--tags', check: false)
+  if git_version.returncode() == 0
+    intel_driver_git_version = git_version.stdout().strip()
+  endif
 endif
 
 version_cfg = configuration_data()