[gnutls-devel] configure should check for <idna.h>

[Ludovic Courtès]

When GNU libidn isn’t installed, compilation of ‘master’ fails with:

--8<---------------cut here---------------start------------->8---
make[4]: Entering directory '/home/ludo/src/gnutls/+build/src/crywrap'
  CC       crywrap.o
../../../src/crywrap/crywrap.c:35:18: fatal error: idna.h: No such file or directory
 #include <idna.h>
                  ^
compilation terminated.
--8<---------------cut here---------------end--------------->8---

I think configure should check for this header, and possibly bail out if
this is a mandatory feature.  WDYT?

Ludo’.