On some systems, C++ sources in the python package are being built with the C compiler.

[tskisner]

This may be related to issue #3.

Symptom reported:

"... on a recent MacOs (10.10.5) in which I installed a genuine gcc (4.9.2) different from clang (7.0.2)
(while gcc has become an alias for clang without OpenMP on MacOSX).

First, in order to compile the C/C++ code (with gcc) I need to set CFLAGS="-std=c99” before running ‘configure’.
But then this becomes a problem for python during ‘make install’ because I get
clang -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -std=c99 -I/usr/local/lib/python2.7/site-packages/numpy/core/include -I/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c healpy/src/_healpy_pixel_lib.cc -o build/temp.macosx-10.10-x86_64-2.7/healpy/src/_healpy_pixel_lib.o
error: invalid argument '-std=c99' not allowed with 'C++/ObjC++’
"