Enable stack protector in external_deps

[slipher]

Set the -fstack-protector-whatever flag in external_deps/build.sh.

Annoyingly, when building with MinGW this will cause an extra DLL dependency. At least this DLL is (so I believe) flavor-independent, so it shouldn't be too bad. We already ship the libssp DLL with our Windows release builds.

I don't think there is any effect on dynamic dependencies for non-Windows platforms but I should double-check.

I think I will skip it for MSVC packages because I'm not 100% sure it would work correctly and also having to ship the libssp DLL with the MSVC binaries would be annoying. Plus those aren't used in releases.