Small build fixes

[ToxicFrog]

Three changes, take whichever ones you like:

• fix "assignment of readonly variable" error introduced recently when moving from char* to char[]
• disable _FORTIFY_SOURCE in the makefile since it's enabled by default on some systems and slashem9 doesn't build if it's enabled
• fix some of the warnings reported by _FORTIFY_SOURCE

The last of these takes a very blunt-instrument approach (wrap the potentially failing calls in assert); I'm not sure if you want a more sophisticated approach to that.

[moon-chilled]

Thanks! Merged.

I took a slightly different approach to the 'assert' bits (wrapped them around a call to impossible)