macOS: Prevent Xcode 9.3 build warnings
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).
Cross build for windows and OSX, with both autotools and cmake
Use contrib/devtools/security-check.py on the windows binaries to
check that PIE is still enforced, and use file on OSX binaries.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3793