x11-themes/gtk-arc-themes: Try to fix build order bug (D29353)

Uncovered while building with devel/samurai but can probably also
happen with ninja eventually.

samu: job failed: /usr/local/bin/glib-compile-resources --sourcedir=common/gtk-3.0 --target=common/gtk-3.0/gtk-lighter.gresource common/gtk-3.0/gtk-lighter.gresource.xml
common/gtk-3.0/gtk-lighter.gresource.xml: Failed to locate checkbox-checked.png

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-05_21h03m50s/logs/errors/gtk-arc-themes-20210127.log

PR:		254678

x11-themes/gtk-arc-themes/files/patch-common_gtk-3.0_meson.build

@@ -0,0 +1,28 @@
+Try to fix build order bug
+
+samu: job failed: /usr/local/bin/glib-compile-resources --sourcedir=common/gtk-3.0 --target=common/gtk-3.0/gtk-lighter.gresource common/gtk-3.0/gtk-lighter.gresource.xml
+common/gtk-3.0/gtk-lighter.gresource.xml: Failed to locate checkbox-checked.png
+
+--- common/gtk-3.0/meson.build.orig	2021-04-07 08:43:47 UTC
++++ common/gtk-3.0/meson.build
+@@ -42,8 +42,10 @@ gtk3_asset_names = run_command(
+
+ assets_svg = gtk3_ver / 'assets.svg'
+
++gtk3_assets = []
++gtk3_hidpi_assets = []
+ foreach asset : gtk3_asset_names
+-  gtk3_assets = custom_target(
++  gtk3_assets += custom_target(
+     'gtk3-' + asset,
+     input : assets_svg,
+     output : asset + '.png',
+@@ -58,7 +60,7 @@ foreach asset : gtk3_asset_names
+     build_by_default : true
+   )
+
+-  gtk3_hidpi_assets = custom_target(
++  gtk3_hidpi_assets += custom_target(
+     'gtk3-' + asset + '-hidpi',
+     input : assets_svg,
+     output : asset + '@2.png',