Log Message:
Install icons and application resources on all Unix platforms.

As discussed on tech-pkg, icons and application resources should be
installed uniformly on all Unix platforms.  Previously, CMakeLists.txt
excluded these on Darwin but not on other Unix platforms.
Consequently, the PLIST was broken on Darwin.  This was fixed with
upstream commit 21d8800ce613775c995212f61d7e95c2189f6da6, which has
been backported here to v3.8.0.

While here, add two buildlink3.mk files, also discussed on tech-pkg,
that should have been present all along but for some reason were not.