I agree with Ronald: making sure all variables in Makefile provide
values that can work in any environment just because distutils offers an
API to read them would be a major pain.

What is planned is to remove sysconfig from distutils, and have it in
the stdlib, with a private module built when ./configure is called, that
would contain these variables. 

And maybe we could filter out some variables in this process to keep
only the ones that can be used by code in all environments.

IOW, if python-config provides what you need, LINKFORSHARED could be
filtered out so the API cannot return it.