ERT-740: LSF config changes

Have added the CMake options ERT_LSF_LIB_PATH and ERT_LSF_INCLUDE_PATH
as hints to the find_library() and find_header() functions searching for
the LSF header LSF library.
Merge pull request #599 from joakim-hove/ERT-740-LSF-config

ERT-740: LSF config changes