AC_CHECK_HEADERS_ONCE: hoist cache name computation to m4 time

AC_CHECK_HEADERS_ONCE: hoist cache name computation to m4 time

Rather than perform a sed script on each element of the
$ac_header_c_list to compute the corresponding cache name, we
can inline enough of AC_CHECK_HEADER to bypass the normal
polymorphic code, and instead directly use the literal
header and cache name that we are consuming from the list.

The resulting configure script is roughly unchanged in size,
but performs slightly faster.

* lib/autoconf/headers.m4 (AC_CHECK_HEADER_COMPILE): Split out shell
function registration...
(_AC_CHECK_HEADER_COMPILE_FN): ...to here.
(_AC_HEADERS_EXPANSION): Use it to inline enough of AC_CHECK_HEADER
to operate on a literal rather than a shell variable, for fewer sed
calls.

Signed-off-by: Eric Blake <eblake@redhat.com>