Make it clear that EIGEN3_INCLUDE_DIR should be passed to cmake, not be set as an env variable

[SatjaSivcev]

From @dragly :

# TODO Make it clear that EIGEN3_INCLUDE_DIR should be passed to cmake, not be set as an env variable
message(FATAL_ERROR "Please point EIGEN3_INCLUDE_DIR to the root directory of your Eigen3 installation (the folder containing Eigen/Core, Eigen/Dense etc.), or disable the Eigen samples (-DUSE_EIGEN3=OFF).")

if(USE_EIGEN3)
    if(NOT DEFINED EIGEN3_INCLUDE_DIR)
    else()
        if(NOT IS_DIRECTORY "${EIGEN3_INCLUDE_DIR}/Eigen")