A) gcc which is a very portable, ISO-C compatible c compiler. Gcc is open source software and available for most platforms. For Linux it is usually part of your distribution, for Windows the McStas distribution package includes a version of gcc (in the Dev-CPP sub-package), and for Mac OS X gcc is part of the Xcode tools package available on the installation medium.
B) icc or the Intel c compiler is available for Linux, Mac OS and Windows systems and is a commercial software product. Generally, simulations run with the Intel compiler are a factor of 2 faster than the identical simulation run using gcc. To use icc with McStas on Linux or Mac OS X, set the environment variables
MCSTAS_CFLAGS="-g -O2 -wd177,266,1011,181"
On Unix (and Mac OS X) systems, to use icc with MPI installations, either
On Windows, the Intel c compiler is 'icl', not 'icc' and has a dependency for Microsoft Visual C++. If you have both these softwares available, running McStas with the Intel compiler should be possible (currently untested by the McStas developer team).