denote that the package uses libXaw.
Introduce an XAW_TYPE definition, which is defined in /etc/mk.conf by
the user, to denote the type of libXaw to use. Possible values are:
standard, 3d and xpm.
Define USE_XAW in relevant Makefiles.
Set default XAW_TYPE in Makefiles which previously did a DEPENDS+= Xaw3d...
This means that package builds should not fail because of conflicting
Xaw packages being installed on machines.
