1/ Check the version using 'find_package_handle_standard_args' internal
checking of version, instead of a home-made check.
2/ No need to add EIGEN3_INCLUDE_DIR in the cache, as find_path does it.
3/ Call 'find_package_handle_standard_args' even if EIGEN3_INCLUDE_DIR is
already and valid in the cache.
- proper indentation
- merge double occurences
- standardize location at the end block, before DOC
tested with GMP(XX), MPFR, MPFI, LEDA, RS(3), NTL and a (small) number of ways to specify library
locations via environment and CMake variables, on Debian 64bit