cmake: Introduce boost path+somme path+MYENV corection
This commit is in the continuation of GemUtil commit 12aa2c6. It use a BOOST_SPECIAL_INCLUDE_PATH variable in Xfem and SplitMesh/test to force usage of a boost library in a non conventional location. This have to be spread all around in eXlibris. It replace properly what was done in eXlibrisType. Boost version checking mechanism may have to be adapted. Somme path are added to path (mainly DEVROOT/External/ include and lib) For MYENV there was missing path in previous commit on the subject.