replace CMAKE_CURRENT_SOURCE_DIR by CMAKE_SOURCE_DIR

replace CMAKE_CURRENT_SOURCE_DIR by CMAKE_SOURCE_DIR

This is an approach to use CMAKE_CURRENT_SOURCE_DIR intead of CMAKE_SOURCE_DIR.

https://cmake.org/cmake/help/latest/variable/CMAKE_CURRENT_SOURCE_DIR.html
http://johnnado.com/cmake-directory-variables/
http://fujii.github.io/2015/10/10/cmake-best-practice

So its possible add libpcap as subdirectory ( https://cmake.org/cmake/help/v3.0/command/add_subdirectory.html ) in other "cmake-projects".