This PR solves #12867 (needs to run autogen.sh && ./configure) clang (Apple LLVM version 9.1.0 (clang-902.0.39.1)) warns unused argument '-pie' during compilation. So we check for warnings in the test using $CXXFLAG_WERROR. Windows is also default-pie and was special-cased because it also warned, but we can also eliminate that case if warnings are caught.
Backport of core PR12899
Depends on D3792 (for the test plan).