Install issues on Linux

[kenavolic]

Hi,

Context: PlusLib install with conan
Platform: Ubuntu 18.04
CMake version: 3.21.1

When installing vtkAddon, there was a missing target file in the cmake install tree.
Moreover ${CMAKE_CURRENT_SOURCE_DIR} should be preferred to ${CMAKE_SOURCE_DIR} as the project could be installed as a subproject and ${CMAKE_SOURCE_DIR} will point to the top level script directory.

PR #24 fixes these issues.

Best Regards,

[brandus1]

can confirm @kenavolic last commit solves the installation issue on ubuntu 20.04