Wolf macro cmake conftime

This pr add a config.h file (template at src/internal/config.h.in) which is set at cmake configure time - before building.

This allows to define/set pre-processing macro before compilation.

A simple example is also available here.

For instance it allows enabling / disabling _WOLF_DEBUG token.

src/examples/CMakeLists.txt

@@ -7,6 +7,7 @@
 # Complete Wolf tree skeleton with smart pointers
 ADD_EXECUTABLE(test_sh_ptr test_sh_ptr.cpp)
 
+ADD_EXECUTABLE(test_wolf_root test_wolf_root.cpp)
 
 IF(Ceres_FOUND)
     # test_processor_odom_3D