For Issue17086, 8ee6d96a1019 backported some cross-build patches to the 2.7 branch.  The changes to setup.py detect_modules differ in the backport from those in default. In particular, in default, changes to the library and include directory lists used to build extensions modules are only made conditionally when cross-compiling.  But the 2.7 backport makes these changes unconditionally.

default:
    def detect_modules(self):
        # Ensure that /usr/local is always used, but the local build
        # directories (i.e. '.' and 'Include') must be first.  See issue
        # 10520.
        if not cross_compiling:
            add_dir_to_list(self.compiler.library_dirs, '/usr/local/lib')
            add_dir_to_list(self.compiler.include_dirs, '/usr/local/include')
        # only change this for cross builds for 3.3, issues on Mageia
        if cross_compiling:
            self.add_gcc_paths()
        self.add_multiarch_paths()

2.7:
    def detect_modules(self):
        # Ensure that /usr/local is always used
        add_dir_to_list(self.compiler.library_dirs, '/usr/local/lib')
        add_dir_to_list(self.compiler.include_dirs, '/usr/local/include')
        self.add_gcc_paths()
        self.add_multiarch_paths()

This breaks certain universal build combinations on OS X when using SDKs and it is possible it could break builds on other platforms as well.  Substituting the default branch code on 2.7 appears to solve the OS X build problem.  Whether it has any negative impact on other builds, in particular cross-compilations is untested.