I'd make get_config_var('srcdir') to be None for installed systems, because the source tree is not available there.

The advantage of making the value None is that this is easy to test for and would quickly break path manipulation code that tries to construct a path to a specific file in the source tree.  

Making the value to be the name of the directory containing Makefile would still break code that tries to access files relative to the source tree, but could that breakage could be harder to track.

BTW. sysconfig really needs a better description of which variables are supposed to be available, it currently exports every definition in the Makefile and pyconfig.h, and not all of those definitions are usable outside of CPython's build process.