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.

docs/sysvinit/meson.build

@@ -4,6 +4,6 @@ custom_target(
         'README',
         input : 'README.in',
         output : 'README',
-        command : [meson_render_jinja2, config_h, '@INPUT@', '@OUTPUT@'],
+        command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],
         install : conf.get('HAVE_SYSV_COMPAT') == 1,
         install_dir : sysvinit_path)