Log Message:
Fix MAKEFLAGS handling to work with spaces in UNPRIVILEGED_GROUPS

Let environment variables handling passing MFLAGS/MAKEFLAGS to children
rather than do it explicitly on the command line.  As of 1.24 of
mk/unprivileged.mk pkgsrc puts UNPRIVILEGED_GROUPS in MAKEFLAGS.
UNPRIVILEGED_GROUPS contains a space-separated list of the groups
the current account is a member of.  This requires proper quoting
in MAKEFLAGS if the current account is a member of more than one group
(due to the spaces), and the old version of Makefile does not take
that into account.