Changes needed to support builds with specific CPU instruction sets

Added new configuration parameter LCG_INSTRUCTIONSET to configure with specific instruction sets. For example -DLCG_INSTRUCTIONSET=avx2. In this case all the packages with the option BUILD_WITH_INSTRUCTION_SET 1 in LCGPackage_Add(...) will be built using a compiler wrapper that adds the compiler option -mXXX for the instruction set. The platform name will be composed accordingly including the supported instruction set (for example x86_64+avx2-mac1013-clang90-opt). For the packages without BUILD_WITH_INSTRUCTION_SET 1the platform name will be the current one without the instruction set. The LCG views are created combining both platform names. For the time being only the packages Vc, VecCore, VecGeom and Eigen are affected by the option BUILD_WITH_INSTRUCTION_SET.