build: avoid rebuilds of unset VARIANT packages

build: avoid rebuilds of unset VARIANT packages

If a Makefile defines some packages with VARIANT set, and others without
it, the latter will be built once for every different VARIANT set, each
build trumping the previous one.

Avoid rebuilds by only building unnamed variant packages when the first
variant is built.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>