Adds support for the library_dirs key, which appears in the documentation but
was never actually implemented. 
library_dirs is essentially equivalent to adding "-Lfoo" to ldflags in Make,
modifying LIBRARY_SEARCH_PATHS in Xcode, or adding AdditionalLibraryPaths to the
VCLinkerTool or VCLibrarianTool in MSVS. I have attempted to make an analogous
change to the ninja generator, but have not been able to test it (ninja was not
in the tree I checked out) and I am fairly sure it's wrong. Guidance here would
be appreciated. 

I have created a modified version of the mac/libraries test to test this change;
a cross-platform test proved impossible because I needed to copy the lib
somewhere, and doing so with a copies section caused a circular dependency which
caused Make to fail (though it worked fine in Xcode). For some reason, although
the implementation of library_dirs is a straightforward addition to
LIBRARY_SEARCH_PATHS, simply substituting the former for the latter did not work
until I removed the <(DEPTH) reference from the library_dirs list. This may
indicate a bug in my code, but I'm not certain, since neither
test/mac/library_dir nor test/mac/libraries worked for me when I tried building
their gypfiles directly and building the products in Xcode.




git-svn-id: http://gyp.googlecode.com/svn/trunk@1657 78cadc50-ecff-11dd-a971-7dbc132099af