configure: don't warn SDL abi if disabled

SDL has the same problem as GTK that we might get warnings on SDL ABI
version even if SDL is disabled.  Fix that by only probing SDL if SDL is
enabled.  Also this should let configure be a little bit faster since we
don't really need to probe SDL stuff when it's off.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Gerd Hoffmann <kraxel@redhat.com>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Daniel P. Berrange <berrange@redhat.com>
CC: Fam Zheng <famz@redhat.com>
CC: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20180410054034.20479-1-peterx@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

configure

@@ -2835,6 +2835,9 @@ fi
 # Look for sdl configuration program (pkg-config or sdl-config).  Try
 # sdl-config even without cross prefix, and favour pkg-config over sdl-config.
 
+sdl_probe ()
+{
+  sdl_too_old=no
   if test "$sdlabi" = ""; then
       if $pkg_config --exists "sdl2"; then
           sdlabi=2.0
@@ -2871,13 +2874,13 @@ else
       feature_not_found "sdl" "Install SDL2-devel"
     fi
     sdl=no
+    # no need to do the rest
+    return
   fi
   if test -n "$cross_prefix" && test "$(basename "$sdlconfig")" = sdl-config; then
     echo warning: using "\"$sdlconfig\"" to detect cross-compiled sdl >&2
   fi
 
-sdl_too_old=no
-if test "$sdl" != "no" ; then
   cat > $TMPC << EOF
 #include <SDL.h>
 #undef main /* We don't want SDL to override our main() */
@@ -2919,6 +2922,10 @@ EOF
     fi
     sdl=no
   fi # sdl compile test
+}
+
+if test "$sdl" != "no" ; then
+  sdl_probe
 fi
 
 if test "$sdl" = "yes" ; then