Revert "Fix assumption that CMAKE_INSTALL_*DIR paths are relative."

[ahcorde]

Reverts #90

This PR is breaking CI

10:46:01 CMake Error at CMakeLists.txt:39 (file):
10:46:01   file RELATIVE_PATH must be passed a full path to the directory:
10:46:01   lib/urdfdom_headers/cmake
10:46:01 
10:46:01 
10:46:01 CMake Error at CMakeLists.txt:40 (file):
10:46:01   file RELATIVE_PATH must be passed a full path to the directory: lib
10:46:01