Add mechanisms to the main cmake tree for building zend extension DSOs in hphp/test/ext_zend_compat.

The .skipif mechanisms for tests was attractive, but didn't work
because foo.php and foo.php.skipif are evaluated using the same args,
so you can't execute foo.php.skipif expecting to build something specified
in the args that is really only needed for foo.php

I do not really claim to know cmake, so cargo-cult programmed these
cmake additions.