CI: Add more compiler warnings

Created by: germasch

@chuckatkins: Learning from the last minute MSVC troubles before the release, which were (in part) caused by variable length arrays on the stack, which is not a standard C++ feature, how about adding -pedantic-errors to the CI when the compiler is gcc/clang? This will catch this problem, and potentially avoid other non-standard use of C++.

It actually turns out that you'd have to add -Wno-gnu-zero-variadic-macro-arguments since gtest relies on that, and no fix appears to exist (yet): https://github.com/google/googletest/issues/1419

A more limited approach would be to just avoid the use of variable length arrays with -Werror=vla.