From https://lists.macosforge.org/pipermail/macports-dev/2011-July/015263.html:
5) Building with a compiler that doesn't support newer __builtins
If your port uses MacPorts compilers rather than the default compiler,
you may run into trouble with string functions. You'll see errors at
link time about undefined __builtin_* functions. If this happens, you
may want to compile with -D_FORTIFY_SOURCE=0 to tell the headers to
use unfortified versions which do not use compiler builtins.
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