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.

sysctl.d/meson.build

@@ -16,7 +16,7 @@ custom_target(
         '50-coredump.conf',
         input : '50-coredump.conf.in',
         output : '50-coredump.conf',
-        command : [meson_render_jinja2, config_h, '@INPUT@', '@OUTPUT@'],
+        command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],
         install : conf.get('ENABLE_COREDUMP') == 1,
         install_dir : sysctldir)