Fix the clang build

Fix the clang build

Simon pointed out that gdb would not build with clang, due to the
addition of -Wimplicit-fallthrough.  This patch fixes the problem by
using -Wimplicit-fallthrough=3 -- this does not work with clang,
bypassing the issue.

Tested by rebuilding with both gcc and clang; and also by verifying
that -Wimplicit-fallthrough=3 is used in the gcc build.

I will file a follow-up bug to convert the fall-through comments to a
form that can be used by both clang and gcc.

gdb/ChangeLog
2018-05-10  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* warning.m4 (AM_GDB_WARNINGS): Use -Wimplicit-fallthrough=3.

gdb/gdbserver/ChangeLog
2018-05-10  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.