Log message:
Reorder some lines so that BUILDLINK_USE_BUILTIN.<pkg> set in the
environment overrides all other settings.

Log message:
LIBTOOL_OVERRIDE and SHLIBTOOL_OVERRIDE are now lists of shell globs
relative to ${WRKSRC}.  Remove redundant LIBTOOL_OVERRIDE settings that
are automatically handled by the default setting in bsd.pkg.mk.

Log message:
USE_BUILDLINK should be "yes", not just defined.

Log message:
remove unneeded CONFIG_{GUESS,SUB}_OVERRIDE, as bsd.pkg.mk does this
automatically now.

Log message:
Create a new variable PREFER_NATIVE that has the opposite semantics
as PREFER_PKGSRC.  Preferences are determined by the most specific
instance of the package in either PREFER_PKGSRC or PREFER_NATIVE.  If
a package is specified in neither or in both variables, then PREFER_PKGSRC
has precedence over PREFER_NATIVE.

Log message:
Reorganize code so that any dependencies are checked as part of deciding
whether the software is built-in or not.  This facilitates implementing
the forthcoming PKGSRC_NATIVE variable.

Log message:
Move the INCOMPAT_FOO checks to a more natural location within the
block that decides whether package FOO is built-in or not.  If the
platform is listed in IMCOMPAT_FOO, then treat FOO as being not
built-in.

Log message:
If we're passing through MAKEFLAGS variables whose values may contain
spaces, use the :Q modifier instead of double-quoting the value.  This
avoids breakage when executing the just-in-time su targets.

Log message:
Make PREFER_PKGSRC just yes/no or a list of packages.  This makes it
simpler to understand.

Log message:
Rename BUILDLINK_PREFER_PKGSRC to PREFER_PKGSRC so that we can use its
value outside of buildlink-related files.