MSVC does not accept -std=c++11 but instead accepts /std:c++11. We cannot
use that however because with /std:c++11 compilation fails with

    .\golang/libgolang.h(280): error C7555: use of designated initializers requires at least '/std:c++20'
    .\golang/libgolang.h(294): error C7555: use of designated initializers requires at least '/std:c++20'
    .\golang/libgolang.h(308): error C7555: use of designated initializers requires at least '/std:c++20'

for

    276 // _selsend creates `_chansend(ch, ptx)` case for _chanselect.
    277 static inline
    278 _selcase _selsend(_chan *ch, const void *ptx) {
    279     _selcase _ = {
    280         .ch     = ch,			<-- NOTE
    281         .op     = _CHANSEND,
    282         .flags  = (enum _selflags)0,
    283         .user   = 0xff,
    284     };

and similar places.

-> Use /std:c++20 instead as MSVC advices  (WOW designated initializers are C99
and it took 21 years for C++ to catch up).

We also need to adjust exception handling mode into "fully conformant" instead
of "assume extern C cannot throw" setuptools default so that `panic` and other
extern C libgolang functions generate exceptions that could be caught
instead of crashing programs.

For C vs C++ flags we also need to adjust our build_ext, because the way
setuptools/distutils organize compilation for MSVC is different from
UNIX compilers.