build: construct path to config.h manually

meson reports:

  internal/meson.build:25: DEPRECATION: Project uses feature that was
  always broken, and is now deprecated since '1.3.0': str.format: Value
  other than strings, integers, bools, options, dictionaries and lists
  thereof.

Thus just hardcode the config file path. The meson issues on this topic
suggest to use current_build_dir but this would change the relative path
to an absolute one. Not sure if this is what we want/need. The incorrect
version produced the same output, so let's play safe and just create the
same path. No one complained in the last two years about this.

Signed-off-by: Daniel Wagner <[email protected]>

internal/meson.build

@@ -22,7 +22,7 @@ config_dep = declare_dependency(
 
 add_project_arguments(
     [
-        '-include', '@0@'.format(config_h),
+        '-include', 'internal/config.h',
     ],
     language : 'c',
 )