Fix undefined "_environ" symbol link MacOSX error specific to shared build.
Fix undefined "_environ" symbol link MacOSX error specific to shared build.

This commit fix the MacOSX shared build that was broken as a side effect
of the removal (commit 375f0fca) of the setresgid/setresuid hack.

thewtex/python-cmake-buildsystem,thewtex/python-cmake-buildsystem,python-cmake-buildsystem/python-cmake-buildsystem,msmolens/python-cmake-buildsystem,adrianbroher/python-cmake-buildsystem,davidsansome/python-cmake-buildsystem,chuckatkins/python-cmake-buildsystem,jcfr/python-cmake-buildsystem