build: disable -Wimplicit-fallthrough warning with clang

Seems clang 10 got support for -Wimplicit-fallthrough, but does
not honor the code comments to suppress the warning. What a
disaster.

Try to detect it.

See-also: https://github.com/ClangBuiltLinux/linux/issues/   #636
See-also: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e2079e93f562c7f7a030eb7642017ee5eabaaa10

m4/compiler_options.m4

@@ -70,7 +70,6 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		      -Wfloat-equal \
 		      -Wformat-nonliteral \
 		      -Wformat-security \
-		      -Wimplicit-fallthrough \
 		      -Wimplicit-function-declaration \
 		      -Winit-self \
 		      -Wlogical-op \
@@ -137,6 +136,26 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		[G_DEFINE_TYPE (NMObject, nm_object, G_TYPE_OBJECT)]
 	)
 
+	dnl clang started supporting -Wimplicit-fallthrough, but it does not
+	dnl honor the code comments to suppress the warning. Disable the
+	dnl warning with clang.
+	dnl
+	NM_COMPILER_WARNING([$1], [implicit-fallthrough],
+		[int foo(int a);
+	     int foo(int a) {
+		    int r = 0;
+		    switch (a) {
+		    case 1:
+		       r++;
+		       /* fall-through */
+		    case 2:
+		       r++;
+		       break;
+		    }
+		    return r;
+		 }]
+	)
+
 	eval "AS_TR_SH([$1])='$CFLAGS_MORE_WARNINGS $$1'"
 else
 	AC_MSG_RESULT(no)

meson.build

@@ -179,7 +179,6 @@ if nm_debug
     '-Wfloat-equal',
     '-Wformat-nonliteral',
     '-Wformat-security',
-    '-Wimplicit-fallthrough',
     '-Wimplicit-function-declaration',
     '-Winit-self',
     '-Wlogical-op',
@@ -211,6 +210,28 @@ if nm_debug
     '-Wunknown-attributes',
     '-fno-strict-aliasing',
   ])
+
+  if cc.has_argument('-Wimplicit-fallthrough')
+    if cc.compiles('''
+                   int main(int argc, char **argv) {
+                       int r = 0;
+                       switch (argc) {
+                       case 0:
+                           r++;
+                           /* fall-through */
+                       case 1:
+                           r++;
+                           break;
+                       }
+                       return r;
+                   }
+                   ''',
+                   args: '-Werror=implicit-fallthrough',
+                   name: '-Werror=implicit-fallthrough')
+      common_flags += '-Wimplicit-fallthrough'
+    endif
+  endif
+
 endif
 
 add_project_arguments(common_flags, language: 'c')