configure: stop user enabling plugins on Windows for now

There are some patches on the list that enable plugins on Windows but
they still need some changes to be ready:

  https://patchew.org/QEMU/20201013002806.1447-1-luoyonggang@gmail.com/

In the meantime lets stop the user from being able to configure the
support so they don't get confused by the weird linker error messages
later.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20210709143005.1554-32-alex.bennee@linaro.org>

configure

@@ -708,6 +708,7 @@ MINGW32*)
     audio_drv_list=""
   fi
   supported_os="yes"
+  plugins="no"
   pie="no"
 ;;
 GNU/kFreeBSD)
@@ -1522,7 +1523,11 @@ for opt do
   ;;
   --disable-xkbcommon) xkbcommon="disabled"
   ;;
-  --enable-plugins) plugins="yes"
+  --enable-plugins) if test "$mingw32" = "yes"; then
+                        error_exit "TCG plugins not currently supported on Windows platforms"
+                    else
+                        plugins="yes"
+                    fi
   ;;
   --disable-plugins) plugins="no"
   ;;