configure: Allow automatic WHPX detection

The whpx variable is currently initialized to "no" which causes the WHPX
check to skip the detection unless the user specified --enable-whpx.
Since the detection code should be able to figure it out correctly, let's
initialized the variable to "" on MinGW-builds for proper auto-detection
instead.

Message-Id: <20200804170055.2851-11-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20200823111757.72002-7-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -849,6 +849,7 @@ case $targetos in
 MINGW32*)
   mingw32="yes"
   hax="yes"
+  whpx=""
   vhost_user="no"
   audio_possible_drivers="dsound sdl"
   if check_include dsound.h; then