Fix gcc version check

* Update hhvm-9999 to use pkg_pretend for the gcc version, rather than having it in RDEPEND (note, gcc should never be have been a **runtime** dependency, as it's only ever used for compiling).
* Include a case statement for easier reuse of ebuilds
* Comment with link to issue explaining why we aren't fetching a tarball for stable releases (because it's very unusual)
* Create hhvm-3.2.0 from the 9999 ebuild (fixes #14)
HHVM requires specific minimum version of cmake, see https://github.com/facebook/hhvm/issues/1306