Replaced /usr/local with the use of ${PREFIX} in calc Makefiles.
The ${PREFIX} is not the same as ${T}. The ${T} specifies
a top level directory under which calc installs things.
While usally ${T} is empty, it can be specific path
as if calc where "chrooted" during an install.
The ${PREFIX} value, during install, is a path between
the top level ${T} install directory and the object
such as an include file.
Corrected a few more typos in Makefile comments.