configure: reenable opengl by default

Because the opengl library is only linked to for the lm32 target, we can
now safely enable opengl by default again.
Signed-off-by: NMichael Walle <michael@walle.cc>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

configure

@@ -177,7 +177,7 @@ spice=""
 rbd=""
 smartcard=""
 smartcard_nss=""
-opengl="no"
+opengl=""
 
 # parse CC options first
 for opt do