Unset MACOSX_DEPLOYMENT_TARGET for all cmake

[certik]

This is needed, otherwise cmake fails to compile packages.

[ahmadia]

This is certainly "new". This is sometimes needed to make Python packages build correctly, and for enforcing deployment targets.

[certik]

There might be some way to convince CMake to work with MACOSX_DEPLOYMENT_TARGET. I found, as you said, that Python (and perhaps more packages) require MACOSX_DEPLOYMENT_TARGET to be properly set so I set it in PROLOGUE. However, cmake always fails to build other packages, if this is set --- so one solution is to unset MACOSX_DEPLOYMENT_TARGET just for packages build using cmake, the other solution is to figure out how to make cmake work with it (I think that would be preferable, probably). This PR implements the former.

[johannesring]

See also issue #368.

[certik]

See also #902. It looks like on El Capitan at least, things are working as expected.