The next patch will add `go` to Pyx API and correspondingly C, C++ and Pyx
level tests for it that will go into _golang_test.pyx extension. For
C-level test we'll use C source file - to verify that libgolang.h could
be used by C projects with C compiler, while for C++ and Pyx-level tests
the sources will be in C++. Thus _golang_test.so will be build from both
C and C++ sources.

This creates a problem: distutils / setuptools use the _same_ compiler
to compile both C and C++ sources and only use C++ compiler at link
stage. Thus, as it is not possible to tune compiler that is used only
for C++ sources, and also as it is not possible to provide per-source
flags, when compiling C-level test, the compiler will be invoked with
`-std=c++11` option that we inject. Gcc tolerates that and only prints a
warning, but Clang considers that an error and gives:

	error: invalid argument '-std=c++11' not allowed with 'C'`

A proper fix would be to change the build system from distutils to
something more flexible that uses C++ compiler for C++ sources and C
compiler for C sources and allows to tune per-unit flags. However it is
not a small step at this stage.

-> Use workaround and tweak options that are used when compiling sources
depending on whether it is C or C++.