GCC User Guide Update

[qutorial]

In the mbeddr user guide in the section to install GCC for Mac:

On the Mac, you should install XCode to get \ic{gcc}, \ic{gdb}, \ic{make} and the associated tools.

is problematic somewhat.

1. On older OS X'es it is hard, as new XCode requires new operating systems. Something like this: https://github.com/kennethreitz/osx-gcc-installer/ could be researched to work. Brew could be used.
2. Mac OS X 10.7 on uses clang, not gcc.

It needs to be taken care of in the manual.

Assigned to Markus, as he said to be rewriting the user guide anyways :)