Some distros enable _FORTIFY_SOURCE by default which upsets building
newlib which itself implements the logic for this define. For example,
building gets.c fails because the includes set up a gets() macro which
expands in the definition.
Since newlib isn't prepared to build itself with _FORTIFY_SOURCE, and
it's not clear if it's even useful, ignore it when building the code.
This also matches what glibc is doing.
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