[RFC] build: turn down the _FORTIFY_SOURCE level on gcc >= 4.0

We probably don't need to worry about it at the moment.  There had been some mention of using C++ at some point, so I included it.  I can back that part out, if you'd rather we didn't have it.  It doesn't affect anything right now.  And this block is only for GNU compilers, so MSVC is unaffected.