Fix Linux build issues

Switched from -msse2 switches to -march=native to allow for non standard cpus to build properly. Tested on Ubuntu 12.0.4 with an ARM processor.

This however will not solve the issue with users building on Digitalocean VPSs. To fix that issue they will have to run the following commands on build:
qtchooser -run-tool=qmake -qt=qt4 "USE_UPNP=-" 
Then modify the makefile to switch -march=native to -march=core2