AC_REPLACE_FUNCS: allow split lines again

AC_REPLACE_FUNCS: allow split lines again

More fallout from the same 2.66 regression that was partially fixed
yesterday, but until yesterday's AS_LITERAL_IF actually treated \ as a
non-literal, apparently no one noticed the latent bug of a spurious
call to AC_CHECK_FUNC([\]) during automake's libobj5.test.

* lib/autoconf/functions.m4 (AC_REPLACE_FUNCS): Flatten newlines
and move guts...
(_AC_REPLACE_FUNCS): ...to new helper.
* tests/semantics.at (AC_REPLACE_FUNCS): Enhance test.
Reported by Ralf Wildenhues.

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