EZC: allow dynamically loadable Zend compat extensions

The first commit is a duplicate of #2608. Only the second commit is new.

* Use the existing macro ZEND_GET_MODULE() to provide the necessary getModule() function.
* In CMake, add macro HHVM_COMPAT_EXTENSION(), equivalent to HHVM_EXTENSION(), which will register a zend-compat extension.
* Set the source language, so that it's not even necessary to rename the files. This makes it trivial to have a dual-targeted source tree.