'make' generates 4 executables

After the recent Makefile changes (which makes things much nicer), the first run of 'make' generates 'wasm' and 'unopt'.  The second run adds 'main.native' and 'main.d.byte'.  Ideally, 'make' would only generate 1 executable.  For ease of use, maybe this should be the unopt build and you have to type 'make opt' to get the opt build?
