bsd.prefs.mk include order

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
as Al asked for it, this is to provide means of documenting specific
order constraints. The following variables currently have to be defined
before including bsd.prefs.mk and why.

PKG_DESTDIR_SUPPORT: otherwise _USE_DESTDIR can't be provided early to
change system logic
PKG_INSTALLATION_TYPES: similiar reasons
META_PACKAGE: sets PGK_DESTDIR_SUPPORT, so the same arguments apply
SETGIDGAMES: some of the platform mk files don't define GAME_USR etc
otherwise. Should be investigated

The first three are technical reasons and "fixes" would result in
reinventing the wheel of bsd.prefs.mk for little gain. The last one is
somewhat artifical.

It would be nice if someone can prepare a patch for the pkgsrc guide.

Joerg