configure.ac: Drop AC_STDC_HEADERS

configure.ac: Drop AC_STDC_HEADERS

This is obsolete, as the docs say:

 AC_STDC_HEADERS
     Replaced by 'AC_HEADER_STDC'

 AC_HEADER_STDC
     This macro is obsolescent, as current systems have conforming
     header files.  New programs need not use this macro.

Unfortunately dropping it is not properly effective because it's
implied by some other stuff.  We are going to deal with that in a
moment.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>