bpo-36146: Add TEST_EXTENSIONS to setup.py (GH-12129)

bpo-36146: Add TEST_EXTENSIONS to setup.py (GH-12129)

Add TEST_EXTENSIONS constant to setup.py to allow to not build test
extensions like _testcapi.

Changes:

* Add add_ldflags_cppflags() subfunction
* Rename add_compiler_directories() to configure_compiler().
* Remove unused COMPILED_WITH_PYDEBUG constant.
* Use self.add() rather than accessing directly self.extensions.
* Remove module_enabled() function: check differently if curses
  extension is built or not.