minor correction to doxygen.make.'s use of add_custom_target().
minor correction to doxygen.make.'s use of add_custom_target().

i modified the way doxygen.cmake invokes add_custom_target() to better conform to how the documentation describes it should be used.

fmrl/ramalloc,fmrl/ramalloc,fmrl/ramalloc,fmrl/ramalloc