Merge pull request #15 from anthonyryan1/master

Fix gcc version check
HHVM 3.2: Use pkg_pretend to check the gcc_version

See d9389358d46e6e6c781e4e21b56474035620a0eb #15 for details.
HHVM: added dependency dev-libs/libzip

See https://github.com/facebook/hhvm/pull/3178 for details.

Fixes #16