makefile corrections

I had to modify your OS X makefile in the following manner to get dna-utils to compile on my 10.9.4 Macbook Air without errors. I would not recommend using g++-mp-{xx} since it denotes specifically the MacPorts version of g++, and some people (like myself) may use other package managers such as Homebrew that do not add a variant suffix such as -mp. 

Also, I'm not sure why -march=native fails on my machine, but I found the solution here: https://stackoverflow.com/questions/10327939/erroring-on-no-such-instruction-while-assembling-project-on-mac-os-x-lion