build: remove _FORTIFY_SOURCE

The vanilla glibc headers produce a warning when _FORTIFY_SOURCE is used
without optimizations [1]. However Debian removes that warning when
packaging glibc [2].

Hardening features should be manually enabled by users.

[1]: https://sourceware.org/git/?p=glibc.git;f=include/features.h;h=glibc-2.42#l435
[2]: https://salsa.debian.org/glibc-team/glibc/-/blob/sid/debian/patches/any/local-revert-bz13979.diff

Origin: #30

Author: MathisMARION

CMakeLists.txt

@@ -112,9 +112,6 @@ if(CC_HAVE_WNO_STRINGOP_OVERFLOW)
     add_compile_options(-Wno-stringop-overflow)
 endif()
 
-# Ask to glibc to check buffer overflows in memcpy() and friends.
-add_compile_definitions(_FORTIFY_SOURCE=3)
-
 # In come case, backtraces are not available without that flags. Increase size
 # of the  binary file, but can be stripped.
 if(CC_HAVE_UNWIND_TABLES)