calc: Simplify and take maintainership

calc provides a horrific Makefile that uses ?= for nothing at all,
requiring the file to be mangled by hand. Still, it was a little
much for us to redefine every system command. cp is unlikely to
wander out of $PATH.

While here, add some options to not install some stuff. In particular,
make it possible to just install a simple usable calculator without
hundreds of scripts, functions, headers, and help files.