Use _FORTIFY_SOURCE

Compiling with -O2 -D_FORTIFY_SOURCE=2 makes glibc functions like memcpy or strcpy do bounds checking for cases where the bounds are known at compile time, such as with statically defined, local buffers.

Higher fortify levels (currently from 1 up to 3) enable stricter behavior and add runtime checks (e.g. if the bounds are known to depend on a variable).

This of requires support for certain compiler builtins and optimization needs to be enabled.