Make our compiler-hardening checks robust against MinGW

Make our compiler-hardening checks robust against MinGW

First, specify -Werror when we are testing each option; if it causes
a warning to appear, we shouldn't be adding it.

Second, do not attempt to add these options until after we have
found the libraries we want.  Previously, I would hit a bug where
the linker hardening options worked fine when we weren't linking
anything, but failed completely once we added openssl or libevent.