Before the path in doxyfile was static and set to "D:/Programmes/mscgen", which obviously won't work on most systems. Now CMake tries to find the tool. If this fails, the path is set empty to let Doxygen look for it inside PATH.
git-svn-id: https://sfml.svn.sourceforge.net/svnroot/sfml/branches/sfml2@1550 4e206d99-4929-0410-ac5d-dfc041789085