CMake DSO build system improvements

The first commit is a duplicate of #2608.

Allow dynamically loadable Zend compat extensions
* 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.

Don't require full source tree for building DSOs:
* Install header files
* Install CMake files needed by hphpize
* Fix hphpize to look for headers and CMake files in the installed directory
* Rename php_config.h-linux to php_config-linux.h and php_config.h-darwin to php_config-darwin.h, so that it will be picked up by *.h glob for installation. Also helps with syntax highlighting.

Also:
* Add an install target to the makefile of DSOs. Install to a version-dependent directory, similar to what PHP does. I chose lib/hphp over lib/hhvm for consistency with include/hphp.