Various Debian packaging fixes

* Bump to debhelper compatibility level 9
* Update Standards-Version to 3.9.5
* Move custom install statements under dh_auto_install to an install file.
* Add to CXXFLAGS but don't override them (+= instead of =).
* Drop libthai0 explicit dependency; shlibs:Depends does this automatically.
* Adjust short and long description based on Debian's description of the
  package.
* Add a Homepage field.

Change-Id: I2eb33c21a42abc6f0ac9a85c0572ebfe240a7bde
Add packaging for the HHVM extension

Add an hhvm-wikidiff2 package and add hacks to build the source twice,
once for PHP and once for HHVM.

Change-Id: I14731220b415c503b78b8df314b7dcc5460d9470