make clean does not work in OpenSource, fails in WebKitLibraries

make clean does not work in OpenSource, fails in WebKitLibraries
https://bugs.webkit.org/show_bug.cgi?id=135734

Patch by Joseph Pecoraro <pecoraro@apple.com> on 2014-08-08
Reviewed by David Kilzer.

Tools:

* Scripts/copy-webkitlibraries-to-product-directory:
Add a --clean switch that removes possible WebKitSystemInterface
and LLVM libraries and headers that may have been copied to the
product directory.

WebKitLibraries:

* Makefile:
Add a "clean" target that runs copy-webkitlibraries-to-product-dir
with the new --clean switch.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@172349 268f45cc-cd09-0410-ab3c-d52691b4dbfc