Compile Cocoa control example in Unix "widgets" sample makefile

Previously this was never done, as we couldn't determine if we were
using Cocoa or not at generation time, but we can determine it in the
makefile itself.

.github/workflows/code_checks.yml

@@ -54,6 +54,7 @@ jobs:
                 ':!build/bakefiles/make_dist.mk' \
                 ':!build/bakefiles/scintilla.bkl' \
                 ':!docs/doxygen/doxygen-awesome-css/*' \
+                ':!samples/widgets/widgets.bkl' \
                 ':!tests/test.bkl' \
                 ':!**/*akefile*' \
                 ':!**/*.pbxproj' \