Python: fix LINKFORSHARED value

LINKFORSHARED incorrectly returns a truncated value by default, which
breaks builds which depend on it returning a valid path.
e.g., it returns:

Python.framework/Versions/2.7/Python

Instead of:

$(brew --prefix)/Python.framework/Versions/2.7/Python

This breaks zpython, and possibly other things. See #20182.

Reported to Python.org in http://bugs.python.org/issue3588 and
http://bugs.python.org/issue16848, fixed for python-config but not
other usecases.