should be pretty safe.
The main risk if is something rely on the 'undef if empty' behavior of
the typical trap is the use of *= in dmake
a git grep of "\$(SYSTEM_" did not reveal any problematic case.

This introduce config_host.mk and config_build.mk
generated from config_host.mk.in
The intent is for that file to be both sourcable in a shell and includable
in a gnu Makefile.
Eventually all the environment set-up will be in config_host.mk.in
and so there won't be a need to source anything in shell before make.

Pointers deleted in destructor are modified in remaining code,
and already set to NULL when necessary.

At least I suppose memory corruption is not intended behavior .-)

Part XIV
Modules
drawinglayer, dtrans, editeng

This partially reverts commit 713e536f .

This reverts commit f3f77fd5.