don't attempt to build p3dembed without p3d_plugin_static

direct/src/plugin_standalone/Sources.pp

@@ -83,6 +83,7 @@
 #end bin_target
 
 #begin bin_target
+  #define BUILD_TARGET $[and $[HAVE_JPEG],$[HAVE_PNG]]
   #define USE_PACKAGES openssl zlib
   #define TARGET p3dembed
   #define LOCAL_LIBS plugin_common p3d_plugin_static