Removed the ${CAL_PASSDOWN}, ${HELP_PASSDOWN}, ${HELP_PASSDOWN},
${CSCRIPT_PASSDOWN} Makefile variables as the new Makefile
include files keep Makefile variables in sync.
Fixed the ability of calc to compile when CUSTOM is undefined
(i.e., -UCUSTOM). The libcustcalc is always built, regardless
of the $(ALLOW_CUSTOM} Makefile variable. However when CUSTOM
is undefined, the bulk of custom functions are not defined.
Dropped support of Makefile.simple and custom/Makefile.simple.
The calc version 2.14.3.0 is the last release that supported
the Makefile.simple and custom/Makefile.simple files.
Now, a make that supports makefile conditional syntax is required.
The platform target section from the old Makefile has been moved
to a new file, Makefile.target. The Makefile.target is shared
by both Makefile and custom/Makefile. The custom/Makefile no
longer includes Makefile (instead it includes ../Makefile.target).
