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)
I'm sorry my branches appear to have become crossed. I hadn't intended to include the the cmake-utils or jemalloc stuff into this pull request. I'll sort that out, sorry to waste your time. I had observed that issue and was looking into ignoring tcmalloc and providing an option to compile with just malloc, but it has not been properly tested yet.