Expose INCLUDEPATH and LIBS qmake variables

This allows one to specify the include and lib paths for gl and zlib.

Example:

    gem install capybara-webkit -- \
      --with-gl-dir=/nix/store/1sw1cyny213ih9dpdsq8h2kwqaqcm6vp-mesa-10.2.9 \
      --with-zlib-dir=/nix/store/cb649pfdf14335d07jcfmsik7a1rsgbf-zlib-1.2.8

Fixes #695