Use a form "#ifndef" more consistently.
The idea is to automatize most of the verification of the consistency
in the names used in the #ifndef and the following #define using a simple :
grep -r -m 1 '#ifndef' */include/CGAL | grep CGAL_ | cut -d' ' -f2 | sort > ifndef.macros
grep -r -m 1 '#define' */include/CGAL | grep CGAL_ | cut -d' ' -f2 | sort > define.macros
diff ifndef.macros define.macros
