At least on Debian requiring version 3.1 of Eigen3 is not sufficient
according to issue #25 the minimum version is 3.1.2. This patch fixes
the required version. In addition it skips the REQUIRED option passed
to find_package(Eigen3) to make the git download work.
Fixes#25.
This define is used by the unit tests; there is no reason why it should
be on the list that must be provided by other projects that use our
headers (the other projects don't use our tests).
This file can be included both in the project itself, and also in the
find module for the library, so that the list of dependencies can be
maintained in only one place.