doxygen.cmake: some fixes
doxygen.cmake: some fixes

Fix the previous commit, which was a patch applied as is from the sofa2
repository.


Former-commit-id: ee51bafb397589781df78a223068d02803465749
hdeling/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,FabienPean/sofa