Add mechanisms to build zend extensions as DSOs, and to test the extension so loaded.

Add a simple zend extension DSO to the new directory hphp/test/ext_zend_compat/hello.

Add a simple unit test that loads this DSO using a .ini file containing just:
  hhvm.dynamic_extensions[hello] = ./test/ext_zend_compat/hello/hello.so
and then executes the php functions implemented by the zend extension.

Extend, as an amateur, the cmake specification files as empirically necessary to get the DSO to build.

The long term goal of this test mechanism is to expose before/after bugs
in the zend compatibility layer, especially in regards to configuration settings.

This new test works, although the extension has been cooked to ensure there's a modify handler
for one of the configuration settings, since otherwise the zend compat layer
seg faults.  (This will be fixed in a future commit.)