meson: also allow setting GIT_VERSION via templates

GIT_VERSION is not available as a config.h variable, because it's rendered
into version.h during builds. Let's rework jinja2 rendering to also
parse version.h. No functional change, the new variable is so far unused.

I guess this will make partial rebuilds a bit slower, but it's useful
to be able to use the full version string.

sysusers.d/meson.build

@@ -33,7 +33,7 @@ foreach tuple : in_files
                 file,
                 input : file + '.in',
                 output: file,
-                command : [meson_render_jinja2, config_h, '@INPUT@', '@OUTPUT@'],
+                command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],
                 install : tuple[1],
                 install_dir : sysusersdir)
 endforeach