gdk: Enable unity builds

Instruct meson to compile all gsk sources in one pass. This lets
the compiler inline freely between our sources, and is much nicer
than moving functions into the headers as static inlines.

gdk/meson.build

@@ -293,11 +293,14 @@ if gdk_backends.length() == 0
 endif
 
 libgdk = static_library('gdk',
+  override_options: [ 'unity=on', 'unity_size=10000' ],
   sources: [gdk_sources, gdk_backends_gen_headers, gdk_gen_headers],
   dependencies: gdk_deps + [libgtk_css_dep],
   link_with: [libgtk_css],
   include_directories: [confinc, gdkx11_inc, wlinc],
-  c_args: libgdk_c_args + common_cflags,
+  c_args: libgdk_c_args + common_cflags + [
+    '-Wno-error=redundant-decls',
+  ],
   link_whole: gdk_backends,
 )