GCC6 for use in parallel with the default compiler

Notice: Prefer to use opt-gcc instead with the compiler version updated in that package

This package installs GCC (C and C++) together with its libraries and headers into /opt/gcc6 . Compiler is configured to use the same ABI as 4.8 series of GCC (-fabi-version=8 --with-default-libstdcxx-abi=gcc4-compatible). This should allow to use the compiled programs on current SFOS and (hopefully?) mix the objects compiled with the different compilers (/opt/gcc6/bin/g++ and /usr/bin/g++).