Don't know why it was missing, build ref_softsdl.so on x86_64 too.

--- Makefile.orig	2007-10-28 21:24:57.000000000 -0200
+++ Makefile	2007-10-28 21:25:20.000000000 -0200
@@ -351,6 +351,10 @@ ifeq ($(ARCH),x86_64)
   $(warning Warning: FXGL not currently supported for $(ARCH))
  endif
 
+ ifeq ($(strip $(BUILD_SDL)),YES)
+  TARGETS += $(BUILDDIR)/ref_softsdl.$(SHLIBEXT)
+ endif
+
  ifeq ($(strip $(BUILD_SDLGL)),YES)
   TARGETS += $(BUILDDIR)/ref_sdlgl.$(SHLIBEXT)
  endif