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.

src/core/meson.build

@@ -151,7 +151,7 @@ load_fragment_gperf_gperf = custom_target(
         'load-fragment-gperf.gperf',
         input : 'load-fragment-gperf.gperf.in',
         output: 'load-fragment-gperf.gperf',
-        command : [meson_render_jinja2, config_h, '@INPUT@', '@OUTPUT@'])
+        command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'])
 
 load_fragment_gperf_c = custom_target(
         'load-fragment-gperf.c',
@@ -217,7 +217,7 @@ foreach item : in_files
                 file,
                 input : file + '.in',
                 output: file,
-                command : [meson_render_jinja2, config_h, '@INPUT@', '@OUTPUT@'],
+                command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],
                 install : (dir == pkgsysconfdir) ? install_sysconfdir_samples : (dir != 'no'),
                 install_dir : dir)
 endforeach