package/ci: fix and clean MinGW Jenkins configuration even more.

Goddamit, who wrote this?!
Require at least CMake 2.8.9.
Use POSITION_INDEPENDENT_CODE CMake property.

Added new BUILD_STATIC_PIC configuration option, PIC is now set only
when needed/requested.
Build test libraries as static.

Removes the need to export symbols for plugins which otherwise don't
need that, we then also don't have to install the test library on
Windows.