Make clean-doc a phony target
Support shell calling convention for make-4.0

There is a difference between make-3.x and make-4.0 on how to call the SHELL.
Previously the SHELL-varible could be set to "/bin/bash --args". This does not
work in make-4.0. Instead .SHELLFLAGS must be set.
Remove usage of TGTDIR in compiler files

Variable TGTDIR is defined in the build makefile but is used in the compiler
makefiles which are included from the main makefile, thus it cannot have been
defined when it was used in the compiler files. Problem detected on switch to
make-4.0.

The usage of TGTDIR in the compiler files was to modify the link flags. The
modification to the link flags is generic and can be/is better solved in the
compile and link target makefile.