make _GNU-ish function declarations visible on cygwin

The lack of this caused several test failures on cygwin64, the one case
I tracked down involved memmem() which is a GNU extension that cygwin
supports.

Since the compiler couldn't see the memmem() prototype it treated it's
return value as int, which was then cast to (char *) preventing any
type-mismatch warning, but since int is 32-bits and (char *) on
cygwin64, the upper 32-bits of the pointer was cleared, resulting in a
crash.

After adding this a test cygwin64 build went from 30 or so test failures
to one.

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