Add Makefile targets for copying static libraries (LLVM and WKSI)

Add Makefile targets for copying static libraries (LLVM and WKSI)
<http://webkit.org/b/132619>

Reviewed by Mark Rowe.

.:
* Makefile:
(MODULES): Add WebKitLibraries.

Tools:
* Scripts/copy-webkitlibraries-to-product-directory: Set a
sensible default for product directory if not specified on
either the command-line or in the environment (by Xcode) by
calling productDir() in webkitdirs.pm.

WebKitLibraries:
* Makefile: Added.
(libs): Install both LLVM and WKSI static libraries.
(all): Add dependency on 'libs' target.
(debug d): Set configuration, the call 'libs' target.
(release r): Ditto.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168410 268f45cc-cd09-0410-ab3c-d52691b4dbfc