configure.ac: configure script does not detect res_ninit and res_ndestroy correctly if resolv.h requires some other header files

[futatuki]

Although the configure.ac checks the header files needed by resolv.h check functions for res_ninit and res_ndestroy does not include them before use resolv.h

This can cause some errors in configure script like issue #203 in trusteddomainproject/OpenDKIM.

With glibc, resolv.h itself contains all declarations for the function, so it is not affected. However in all other platform which requires some other headers for using resolv.h but has res_ninit() and/or res_ndestroy function, the check would be failed.

[thegushi]

I don't know how this would play on BSD (where it's primarily clang compiler) or under weird OSes that use the MUSL support, so I need to QA this a bit and make sure that this fix doesn't make things worse on one of the other platforms.

[futatuki]

Another point of view, configure.ac uses AC_HEADER_RESOLV[1], which checks header files need to include before using resolv.h. So all the tests using resolv.h in configure should also use those include files, for consistensy.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.70/html_node/Particular-Headers.html

[futatuki]

Note: on FreeBSD it is no longer affected since 13.2/14.1 releases, because of those commits for the respection of portabirity from Linux and NetBSD:

freebsd/freebsd-src@4487c62
freebsd/freebsd-src@5a70219
freebsd/freebsd-src@58cf91d

This resolv.h originally came from isc-bind.