[plt-scheme] mzc should honour env vars like CC CXX CFLAGS

An enhancement request, I suppose.

I've just built drscheme using icc, and can't see how to make the 
"make install" step use a different C compiler for building the .so
files.  I know about the ccf switch family, but clearly The Right
Thing is that it should work in the same way that the "make" step, and
indeed everything else, does ;-)

John