Update readme for OS X users

On OS X, I found that without the `-stdlib=libc++` argument, clang throws the following error:

    clang -std=gnu++0x  -include config.h -I. -I.deps -I./tamer  -W -Wall -g -O2 -MD -MF .deps/clp.d -MP -include config.h -c -o clp.o clp.c
    error: invalid argument '-std=gnu++0x' not allowed with 'C/ObjC'

The added specification of a stdlib seems to solve the problem, somehow.