Updated Mac build instructions

[Robert C. Helling]

Henrik,



> Am 26.12.2014 um 11:02 schrieb Henrik Brautaset Aronsen <subsurface at henrik.synth.no>:
> 
> I updated the build instructions on the web page awhile ago,

Do those really work for you? For me qt5 from homebrew is "keg only" i.e. Does not link files not to interfere wit qt4. Even 

brew link --force qt5

Leaves my system in an unusable mess. So I gave up on qt5 from homebrew and compiled it manually. 

Best
Robert