[FindIPOPT.cmake] synch with YCM 0.4.0
[FindIPOPT.cmake] synch with YCM 0.4.0

In particular, add the workaround for https://github.com/robotology/icub-main/issues/418 , that should solve https://github.com/robotology/idyntree/issues/329 as well
robotology/idyntree,robotology/idyntree,robotology/idyntree,robotology/idyntree,robotology/idyntree