Fix _FORTIFY_SOURCE macros

[disconnect3d]

This commit fixes the mistake in the _FORTIFY_SOURCE macro where it was not prefixed with underscore while it has to be (see e.g. https://github.com/search?q=repo%3Abminor%2Fglibc%20FORTIFY_SOURCE&type=code).

Additionally, to make this macro add extra security, one has to enable optimizations. I am not sure if the build system enables them, but it is worth double checking that as well.

Overall, I would recommend using -D_FORTIFY_SOURCE=3 with -O2 or -O3. (The fortify source level 3 was added recently and you can read more about it here: https://developers.redhat.com/blog/2021/04/16/broadening-compiler-checks-for-buffer-overflows-in-_fortify_source).

You can also see the result of the correct vs incorrect macro along with optimizations and no optimizations on this screenshot (source):

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