Get rid of CMake variables assuming Nabla as top level of the source tree

[AnastaZIuk]

Nabla uses many variables referencing to the top level of the source tree, it should not be done this way and if we would like to see Nabla building without External Project we will have to remove them and replace with variables referencing to current scope of handled tree.

[AnastaZIuk]

it also considers build tree