cmake: Fix setting -D_FORTIFY_SOURCE=2.
cmake: Fix setting -D_FORTIFY_SOURCE=2.

mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/libssh