Ensure that sys.prefix can reliably be found

on OSX. This fixes a small issue that was exposed
by running test_subprocess through regrtest (and
hence in a subdirectory).

Without this patch running python.exe from the
build tree will fail when these tree conditions
are true:
1) the CWD is not the root of build tree
2) python.exe is found through $PATH
3) the framework is not yet installed