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: Cole Robinson <crobinso@redhat.com>
Message-id: c9e570b5964d128a3595efe3170129a3da459776.1462557436.git.crobinso@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
