Don't try using -mthreads with clang

This option is useless with it and is, at best, ignored or, at worst,
results in errors if it happens to be used together with -Werror,
-Wunused-command-line-argument

See #23314.

Closes #23317.

configure

@@ -37171,7 +37171,7 @@ else
         x86_64-*-mingw* )
         ;;
         *-*-mingw32* )
-                        { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler supports -mthreads" >&5
+                                                { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler supports -mthreads" >&5
 $as_echo_n "checking if compiler supports -mthreads... " >&6; }
 if ${wx_cv_cflags_mthread+:} false; then :
   $as_echo_n "(cached) " >&6
@@ -37186,6 +37186,10 @@ int
 main ()
 {
 
+#ifdef __clang__
+#error no
+#endif
+
   ;
   return 0;
 }

configure.in

@@ -5251,13 +5251,20 @@ else
         x86_64-*-mingw* )
         ;;
         *-*-mingw32* )
-            dnl check if the compiler accepts -mthreads
+            dnl check if the compiler is gcc and accepts -mthreads (clang just
+            dnl ignores this option but it doesn't need it, so explicitly fail
+            dnl the test for it)
             AC_CACHE_CHECK([if compiler supports -mthreads],
                 wx_cv_cflags_mthread,
                 [
                     CFLAGS_OLD="$CFLAGS"
                     CFLAGS="-mthreads $CFLAGS"
-                    AC_TRY_COMPILE([], [],
+                    AC_TRY_COMPILE([],
+                        [
+#ifdef __clang__
+#error no
+#endif
+                        ],
                         wx_cv_cflags_mthread=yes,
                         wx_cv_cflags_mthread=no
                     )