Hopefully finally fixed constexpr issues in ColorTest with GCC 4.6.

Follow-up to 80b61be4851910d575547516e6ce75b883aca4a4, I am too lazy to
test it before commiting.
Properly fix linking to libdl.

Use CORRADE_PLUGINMANAGER_LIBRARIES instead of
CORRADE_PLUGINMANAGER_LIBRARY when linking core Magnum library.

This reverts commit 80263eb318034e1617e5cf7faddff63f411a0b82 and
6dbe31f0f03856d78b61d7214a3ded49f70010cf.