Fix MACOSX_DEPLOYMENT_TARGET handling. (#1338)
Fix MACOSX_DEPLOYMENT_TARGET handling. (#1338)

Previously it was assumed the MACOSX_DEPLOYMENT_TARGET sysconfig var
was always a string and that has proven to be false.

Fixes #1337
pantsbuild/pex,jsirois/pex,jsirois/pex,pantsbuild/pex