Adding extra args to help with compiling

[rosecers]

Without these args, on OSX Catalina (10.15.4) throws the error:

fsph/../src/spherical_harmonics.hpp:1:10: fatal error: 'iostream.h' file not found
#include <iostream>```

for many "includes" in multiple files. This fix is modeled after a solution from [this random issue discussion](https://github.com/wouterboomsma/eigency/issues/32) which fixed the problem.

[klarh]

I know we've run into this issue on basically all the glotzerlab projects that end up compiling things at one point or another; I don't think unconditionally adding that flag is the right choice because that breaks compilation on linux, though. I think the little if-clause at https://github.com/glotzerlab/freud/blob/deef8b5678e10c1a8157e4b8e617f80aea2f1706/setup.py#L406 fixes this issue in freud; could you try just that change in setup.py and see if that fixes the problem?

[rosecers]

I know we've run into this issue on basically all the glotzerlab projects that end up compiling things at one point or another; I don't think unconditionally adding that flag is the right choice because that breaks compilation on linux, though. I think the little if-clause at https://github.com/glotzerlab/freud/blob/deef8b5678e10c1a8157e4b8e617f80aea2f1706/setup.py#L406 fixes this issue in freud; could you try just that change in setup.py and see if that fixes the problem?

It worked! I included the fix and rebased. CI running now.

[klarh]

Some of the tests would fail because scipy wasn't installed; I rebased your commit to fix that too.