headers in ${BUILDLINK_DIR}, simply create BUILDLINK_CPPFLAGS.<pkg>
variables whose values are appended to CPPFLAGS, which are automatically
passed to the configure and build processes.
BUILDLINK_TRANSFORM.<pkg> has little use in buildlink2 since packages are
now told that the software may be found where it really is installed, not
in ${BUILDLINK_DIR} as was the case with buildlink1. Eventually, these
variables will be declared unsupported by buildlink2.
