Autoconf stanza to test for Newlib?

[Hans-Bernhard Bröker]

Am 08.11.2017 um 01:13 schrieb Jeffrey Walton:
> Hi Everyone,
> 
> We are trying to add Autoool support to an existing library. We don't
> do much of anything special if Newlib is the runtime library. We just
> need to add an _XOPEN_SOURCE=600.

It doesn't make terribly much sense to have to define such a macro only 
for Newlib.  If your code needs that macro for Newlib, it'll need it for 
all libc implementations, or at least all that do know about it.

Given that, I believe you should be looking for

	AC_USE_SYSTEM_EXTENSIONS

instead.