Override wrapped compilers with environment variables

I just upgraded my development machine from Ubuntu 14.04 to Ubuntu
14.10, Doing so upgraded the compiler from GCC 4.8 to GCC 4.9.  I then
discovered, to my chagrin, that DragonEgg doesn't build against GCC
4.9.  (This is with LLVM/Clang/DragonEgg revision 223694.)

This revision of Byfl therefore includes two changes to address this
issue: (1) it produces an error message is build-llvm-byfl is run
against GCC 4.9, and (2) it enables the BF_GCC, BF_GXX, BF_GFORTRAN,
and BF_GCCGO environment variables to override the underlying compiler
used by, respectively, bf-gcc, bf-g++, bf-gfortran, and bf-gccgo.