Add comment for macro get_compiler_family
Fix: NTA_PLATFORM_darwin86 branch and else branch broken after numenta/nupic.core#94 .

By carefully comparing with https://github.com/numenta/nupic.core/blob/2d4d191f10812450a95dd4e4d8492f0d3af79e14/src/main/math/array_algo.hpp