Makefile: make it easy to change warning flags.

Some people need to get rid of "-Werror".
Make that easier wit
  make CWFLAGS=

Reported-by: Christian Sonne
Signed-off-by: NeilBrown <neilb@suse.de>
Makefile: make it easy to suppress the "-D" flag to "install".

Some versions of "install" do not support '-D', and it isn't needed
when installing to default location.
So all
  make INSTALL=install
to suppress the -D

Reported-by: Christian Sonne
Signed-off-by: NeilBrown <neilb@suse.de>