configure: build SDL if only SDL2 available

Right now if SDL2 is installed but not SDL1, default configure will
entirely disable SDL. Check upfront for SDL2 using pkg-config, but
still prefer SDL1 if both versions are installed.
Signed-off-by: NCole Robinson <crobinso@redhat.com>
Message-id: c9e570b5964d128a3595efe3170129a3da459776.1462557436.git.crobinso@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

configure

@@ -207,7 +207,7 @@ fdt=""
 netmap="no"
 pixman=""
 sdl=""
-sdlabi="1.2"
+sdlabi=""
 virtfs=""
 vnc="yes"
 sparse="no"
@@ -2420,6 +2420,16 @@ 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.
 
+if test "$sdlabi" = ""; then
+    if $pkg_config --exists "sdl"; then
+        sdlabi=1.2
+    elif $pkg_config --exists "sdl2"; then
+        sdlabi=2.0
+    else
+        sdlabi=1.2
+    fi
+fi
+
 if test $sdlabi = "2.0"; then
     sdl_config=$sdl2_config
     sdlname=sdl2