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.