Re: link-warning usage improvements

[Bruno Haible]

Eric Blake wrote:
> gl_CHECK_NEXT_HEADERS invokes AC_CHECK_HEADERS under the hood.

Well, it does, but it's not documented. It's an abstraction violation to
exploit this knowledge (even though we do in some places).

> That means several other .m4 file can probably be simplified:

Reducing two invocations of AC_CHECK_HEADERS_ONCE to a single one will
IMO not bring large benefits. I would leave the uses of gl_CHECK_NEXT_HEADERS
alone. Working on the compile-time warning instead of the link warning is
more promising.

Bruno