Fixed build issues on Centos 7.3 clang 3.4 issues

[brendanhoran]

Centos 7.3 uses CLANG 3.4. CLANG 3.4 and below do not understand "fstack-protector-strong".
This patch checks for Centos 7.3 and switches back to GCC.
EG :

# make sdist
python setup.py sdist --formats=zip
clang: error: unknown argument: '-fstack-protector-strong'
clang: error: unknown argument: '-fstack-protector-strong'
clang: error: unknown argument: '-fstack-protector-strong'
clang: error: unknown argument: '-fstack-protector-strong'

Happens no matter what "make option" is used.

[FelixSchwarz]

Your patch does not affect/fix building with "python setup.py build", right?

However there is a workaround:

export LIBSOURCEMAP_MANYLINUX=1