Fix building of specific qt4 ports after qt5 has become the default flavor.

The ports infrastructure does not consistently deal with FLAVOR environment
variables set to an empty string. This caused the dependency check in
portmaster to fail for multimedia/phonon.

With FLAVOR="" in the environment, "make -V FLAVOR" returns "qt4", but the
dependency list produced by "make build-depends-list" assumes "qt5", even
though no user preferences for qt5 have been set.

Reported by: Walter Schwarzenfeld <w.schwarzenfeld at utanet.at>
Approved by: antoine (implicit)