fbcode_builder: support installing thrift generated header files
fbcode_builder: support installing thrift generated header files

Summary:
Update add_thrift_cpp2_library() to add a PUBLIC_HEADER property to the
generated library target, so that the generated headers can be installed using
an `install()` call.  Also add a `HEADER_INSTALL_DIR` property which indicates
the directory they should be installed into.  A `INCLUDE_DIR` argument was
added to customize the include installation path, should anyone care to do so.

This also removes code that was previously incorrectly installing the
generated headers into the source tree.

Reviewed By: wez

Differential Revision: D16853404

fbshipit-source-id: 6f640b2bd347c99804e074fca0209dda37c8a9cf

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro