Log Message:
Support a new global variable:

BUILDLINK_PREFER_PKGSRC
	This variable determines whether or not to prefer the pkgsrc
	versions of software that is also present in the base system.

	This variable is multi-state:
		defined, or "yes"	always prefer the pkgsrc versions
		not defined, or "no"	only use the pkgsrc versions if
					needed by dependency requirements

	This can also take a list of packages for which to prefer the
	pkgsrc-installed software.  The package names may be found by
	consulting the value added to BUILDLINK_PACKAGES in the
	buildlink[23].mk files for that package.