LDCONFIG_ADD_CMD and LDCONFIG_REMOVE_CMD are set by emulator/*.mk
in order to update the library search paths in the emulated system
directories. Substitute for those values, but default them to the
empty string so that the default action taken by the "shlibs" task
can be used.
A full dependency on the "pkgtasks" package will be added if a
non-empty data file is generated by the framework.
This framework is not yet hooked into bsd.pkg.mk, pending some
other changes.