build: Don't use configure_file() when custom_target() works

These two files don't need to be built when configuring, they are only
needed during build.

gsk/meson.build

@@ -97,10 +97,6 @@ if have_vulkan
 endif
 
 gsk_private_vulkan_shaders = []
-# This is an odd split because we use configure_file() below to workaround
-# a limitation in meson preventing using custom_target() with gnome.compile_resources()
-# and that requires file paths, but we also need to have dependencies during development
-# on constantly regenerated files.
 gsk_private_vulkan_compiled_shaders = []
 gsk_private_vulkan_compiled_shaders_deps = []
 gsk_private_vulkan_shader_headers = []
@@ -148,7 +144,7 @@ if get_variable('broadway_enabled')
   ])
 endif
 
-gsk_resources_xml = configure_file(output: 'gsk.resources.xml',
+gsk_resources_xml = custom_target(output: 'gsk.resources.xml',
   input: 'gen-gsk-gresources-xml.py',
   command: [
     find_program('gen-gsk-gresources-xml.py'),

gsk/vulkan/resources/meson.build

@@ -93,12 +93,12 @@ foreach shader: gsk_private_vulkan_shaders
 endforeach
 
 foreach shader: gsk_private_vulkan_vertex_shaders
-  shader_header = configure_file(output: '@0@.h'.format(shader),
-                                 input: shader,
-                                 command: [
-                                   find_program('generate-header.py'),
-                                   '@INPUT@',
-                                 ],
-                                 capture: true)
+  shader_header = custom_target(output: '@0@.h'.format(shader),
+                                input: shader,
+                                command: [
+                                  find_program('generate-header.py'),
+                                  '@INPUT@',
+                                ],
+                                capture: true)
   gsk_private_vulkan_shader_headers += shader_header
 endforeach