configure: force -mconsole when linking SDL under MinGW

configure: force -mconsole when linking SDL under MinGW

When building SDL with MinGW, it sets -mwindows with the
assumption that the application is a GUI application. If this
is linked without passing -mconsole to configure via
--extra-ldflags, stdout will be silenced from cmd.exe while
running FFmpeg.

The -mwindows flag that causes this behavior is included in the
sdl_libs variable, so append -mconsole there rather than create
an sdl_ldflags case just to insert it (especially if -mconsole
must come *after* -mwindows in order to be effective).

Reviewed-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>