Qhull local vs global

[alaingdl]

small issue in CMake file(s) when we do have a global (system) qhull version but we would like to used a local one (e.g. 2020.2)

QHULL_INCLUDE_DIR and QHULL_R_LIBRARY are wrong when providing a path to a local Qhull

cmake [...] -DQHULLDIR=/some/local/path/qhull