ninja: Don't add a new ninja-level variable for libdirs.

$libdirs was always in front of $libs, so there's no need for a second variable.
If $libdirs is empty, the link line no longer contains two spaces, ninja needs
to do fewer variable lookups when parsing .ninja files, and libdirs evaluation
moves to WriteLink() -- there's no need to write libdirs for static libraries
for example.

No intended behavior change.

R=mark@chromium.org

Review URL: https://codereview.chromium.org/18793003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1664 78cadc50-ecff-11dd-a971-7dbc132099af