CI: Add more compiler warnings

[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): google/googletest#1419

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

[chuckatkins]

Another, possibly more robust option, would be to disable building with extensions. The default mode for CMake when specifying language standards is to use the extended flavor, i.e. -std=gnu++11 instead of -std=c++11, since that's what most compilers default to. We should be able to easily change that in the top level CMakeLists.txt for any compiler for which CMake understands (which is most).

[germasch]

I agree that enforcing standard conforming code (ie, C++11) would be good. I'm not sure whether one still has to add -pedantic-errors, anyway.

In practice, it might turn out that one can't really enforce everything strictly, like the gtest limitation I mentioned. But it's certainly worth trying ;)

[chuckatkins]

We can't add pedantic as it will break some of the third party code that we don't have control over but would should be able to enforce c++ standards modes.