Add packaging for the HHVM extension

Add an hhvm-luasandbox package and add hacks to build the source twice,
once for PHP and once for HHVM.

Especially worthy of note is that the luasandbox_version.h generation is
moved from Makefile.frag to debian/rules, as it was depending on Debian
pacakging anyway (debian/changelog) and was incompatible with HHVM.

Change-Id: I742477aead9f137c81fbe01f9a2c23ab0a85c7eb