Don't use -mthreads option when building with MinGW

This option seems to be obsolete and was only ever needed for classic
MinGW32 which is not even supported any more.

This basically reverts a0b9e27fd4 (Applied Fabian Wenzel's patches to
add -mthreads for threads to work in MinGW., 2002-08-28).

See #23314.

Closes #23316.

configure.in

@@ -4240,11 +4240,9 @@ dnl flush the cache
 AC_CACHE_SAVE
 
 dnl ---------------------------------------------------------------------------
-dnl thread support for Unix (for Win32 and OS/2 see past
-dnl                          the next matching "else")
+dnl thread support for Unix (other platforms don't need anything special)
 dnl ---------------------------------------------------------------------------
 
-dnl under MSW (except mingw32) we always have thread support
 if test "$TOOLKIT" != "MSW"; then
 
     dnl the code below:
@@ -4624,37 +4622,6 @@ if test "$TOOLKIT" != "MSW"; then
         fi
       fi
     fi
-dnl from if !MSW
-else
-    if test "$wxUSE_THREADS" = "yes" ; then
-      case "${host}" in
-        x86_64-*-mingw* )
-        ;;
-        *-*-mingw32* )
-            dnl check if the compiler accepts -mthreads
-            AC_CACHE_CHECK([if compiler supports -mthreads],
-                wx_cv_cflags_mthread,
-                [
-                    CFLAGS_OLD="$CFLAGS"
-                    CFLAGS="-mthreads $CFLAGS"
-                    AC_TRY_COMPILE([], [],
-                        wx_cv_cflags_mthread=yes,
-                        wx_cv_cflags_mthread=no
-                    )
-                ]
-            )
-
-            if test "$wx_cv_cflags_mthread" = "yes"; then
-                dnl it does, use it
-                WXCONFIG_CFLAGS="$WXCONFIG_CFLAGS -mthreads"
-                LDFLAGS="$LDFLAGS -mthreads"
-            else
-                dnl it doesn't
-                CFLAGS="$CFLAGS_OLD"
-            fi
-        ;;
-      esac
-    fi
 fi
 
 AC_CHECK_FUNC(localtime_r, [ AC_DEFINE(HAVE_LOCALTIME_R) ])