Add configure check for -Wimplicit-fallthrough

[Jakub Jelinek]

On Tue, Sep 27, 2016 at 03:05:24PM +0200, Marek Polacek wrote:
> Using -Wno-error where only -Wno-implicit-fallthrough was meant was deemed
> to coarse, so this patch attempts to add a configure check for this warnign
> and only use -Wno-implicit-fallthrough when appropriate.
> 
> Bootstrapped on x86_64-linux and ppc64-linux, ok for trunk?
> 
> 2016-09-27  Marek Polacek  <polacek@redhat.com>
> 
> 	* Makefile.in (insn-attrtab.o-warn, insn-dfatab.o-warn,
> 	insn-latencytab.o-warn, insn-output.o-warn, insn-emit.o-warn): Use
> 	@W_NO_IMPLICIT_FALLTHROUGH@ instead of -Wno-error.
> 	* configure.ac: Add check for -Wimplicit-fallthrough.
> 	* configure: Regenerate.

Shouldn't this use ACX_PROG_CXX_WARNING_OPTS ?

	Jakub