CMakeLists.txt: use CMAKE_CURRENT_LIST_DIR for module path

[htfy96]

Fix #301

[bernhardmgruber]

Hi! Thank you for your contribution! There are 4 places using CMAKE_CURRENT_SOURCE_DIR in the main CMakeLists.txt and the VcMacros.cmake file. I would be surprised if you do not need to change all 4 of them. Could you please also change the other instances?
Using CMAKE_CURRENT_SOURCE_DIR in the examples and tests is probably fine, since you don't care about them when using Vc as a submodule.

[amadio]

I had a quick look, and I think the remaining places can actually stay as CMAKE_CURRENT_SOURCE_DIR. We can merge this.