How to check for a #DEFINE?

[Dan Mahoney]

All,

One of OpenDMARC’s users has commented that on the musl c library, the defines 
NETDB_SUCCESS and NETDB_INTERNAL are absent.

While we could add:

+#ifndef NETDB_INTERNAL
+#define NETDB_INTERNAL (-1)
+#endif

To our own code, this feels like something autoconf should be able to test for 
and do for us.

But I can’t seem to figure out how, without running AC_COMPILE_IFELSE.  If 
that’s the answer, I’m willing to try it (or, as above, just add the ifdefine 
to our code), I just want to make sure I’m not missing something obvious.

I’ve had to do some autoconf-bashing since we got a bug report to support 2.70, 
so it’s making me want to understand the WHOLE THING.  Which is turning out to 
be a journey.

-Dan