macOS: Prevent Xcode 9.3 build warnings

Summary:

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
https://github.com/bitcoin/bitcoin/pull/12899/files

Depends on D3792 (for the test plan).

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