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).