Slightly modified the setup.py to support apple silicon build with brew llvm and brew gsl

[quentin-leboutet]

This is just a minor fix to support build on apple silicon. On Mac with Apple silicon, use Homebrew to install the llvm build framework and gsl library:

• brew install llvm
• brew install gsl

This should allow you to set the following in setup.py:

os.environ["CC"] = "/opt/homebrew/opt/llvm/bin/clang"
os.environ["LDFLAGS"] = "-L/opt/homebrew/opt/llvm/lib"
os.environ["CPPFLAGS"] = "-I/opt/homebrew/opt/llvm/include"
gsl_inc = "/opt/homebrew/opt/gsl/include"
gsl_lib = "/opt/homebrew/opt/gsl/lib"

fathon can then be installed by running pip install . within the repository. Tested on an M1 macbook.