Log Message:
Fix build error like:
-- Library config detected: Qt3/CMakeLists.txt
CMake Error at src/CMakeLists.txt:33 (add_subdirectory):
  add_subdirectory given source
  "/usr/tmp/math/cgal/work/CGAL-3.9/src/CGALQt3/CMakeLists.txt" which \ 
is not
  an existing directory.
Call Stack (most recent call first):
  src/CMakeLists.txt:80 (configure_component)

I guess the behavior of string(REPLACE ...) changed with the newer version
of cmake ...