Fix #253

[DaanDeMeyer]

This fixes most of the issues reported in #253. One issue that remains is that DOCTEST_LIBRARY_IMPLEMENTATION in doctest.cpp is reported as unused. I'm suspecting it probably isn't so the solution might be to just ignore the warning.

(Note that I had to remove EXCLUDE_FROM_ALL from doctest_with_main to trigger most of the warnings fixed in this pr).

[onqtam]

Thanks for this PR!

However I'll be able to evaluate it in August - these are some pretty busy weeks for me.

Also this issue seems related:
#243
(I just wanted to point it out)

[DaanDeMeyer]

The cleanest way to solve the C++ version issue in CMake is requiring CMake 3.6 and using target_compile_features(target PUBLIC cxx_std_11). But of course this requires a higher CMake minimum version so its your call.

[DaanDeMeyer]

I tried removing the DOCTEST_LIBRARY_IMPLEMENTATION and got compilation errors as expected so i just ignore the Wunused-macros waring for DOCTEST_LIBRARY_IMPLEMENTATION.

With the last commit, all the issues mentioned in #253 are fixed.

[DaanDeMeyer]

Dropped the specific handling of Clang GNU frontend on Windows since CMake is fixing this itself, see https://gitlab.kitware.com/cmake/cmake/issues/19496.

[onqtam]

I changed the base from master to dev since all PRs should go into dev - I'll look into releasing a new official version soon which will go into master :)