Add implicit fall through warning

Requires modification to our comment about fall through to match compilers
expectations.  Works for GCC and Clang.

Signed-off-by: Allan McRae <allan@archlinux.org>

configure.ac

@@ -462,6 +462,7 @@ if test "x$warningflags" = "xyes" ; then
 	CFLAGS_ADD([-Wformat-nonliteral], [WARNING_CFLAGS])
 	CFLAGS_ADD([-Wformat-security], [WARNING_CFLAGS])
 	CFLAGS_ADD([-Wignored-qualifiers], [WARNING_CFLAGS])
+	CFLAGS_ADD([-Wimplicit-fallthrough], [WARNING_CFLAGS])
 	CFLAGS_ADD([-Winit-self], [WARNING_CFLAGS])
 	CFLAGS_ADD([-Wlogical-op], [WARNING_CFLAGS])
 	CFLAGS_ADD([-Wmissing-declarations], [WARNING_CFLAGS])

meson.build

@@ -220,6 +220,7 @@ if get_option('buildtype').startswith('debug')
     '-Wformat-nonliteral',
     '-Wformat-security',
     '-Wignored-qualifiers',
+    '-Wimplicit-fallthrough',
     '-Winit-self',
     '-Wlogical-op',
     '-Wmissing-declarations',

src/pacman/pacman.c

@@ -196,7 +196,7 @@ static void usage(int op, const char * const myname)
 				addlist(_("      --ignore <pkg>   ignore a package upgrade (can be used more than once)\n"));
 				addlist(_("      --ignoregroup <grp>\n"
 				          "                       ignore a group upgrade (can be used more than once)\n"));
-				/* pass through */
+				/* fall through */
 			case PM_OP_REMOVE:
 				addlist(_("  -d, --nodeps         skip dependency version checks (-dd to skip all checks)\n"));
 				addlist(_("      --assume-installed <package=version>\n"