Fix warning from compiling C++ with a C compiler and invalid flags

[stuartarchibald]

Reporting a bug

• I am using the latest released version of Numba (most recent is visible in
  the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• I have included below a minimal working reproducer (if you are unsure how
  to write one see http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports).

Here:
https://github.com/numba/numba/blob/master/numba/pycc/platform.py#L52-L53
the distutils customized C compiler tries to compile some C++ (denoted by file suffix) with a flag that is invalid for C++ source. Fix it!

[sklam]

What is the invalid flag?

[stuartarchibald]

from memory, think it was -Wstrict-prototypes, which is C/obj-C only.

[99991]

I've got the same issue with numba version 0.44.1 on Debian with g++ version 8.3.0. The error message is:

cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++```