Fix wheezy better

Rely on gcc 4.9 being locally installed and built; pass the right args
to statically link the bits that we care about. Much better than sucking
in a new libc.

Directions for getting 4.9 working on wheezy at
https://github.com/facebook/hhvm/wiki/Building-and-installing-HHVM-on-Debian-7

Fixes https://github.com/hhvm/packaging/issues/87