[qt4-build-multilib.eclass] Always extract doc.pri, qpluginbase.pri, src.pro, tools.pro
lxqt: make policykit dependency optional. add more optional deps to meta
[qt4-build-multilib.eclass] Simplify how toolchain vars are respected.

Backported from qt5-build.eclass
Merge pull request #49 from jauhien/master

lxqt: make policykit dependency optional. add more optional deps to meta