find_package Eigen3 3.x / 5.x compatibility

codes that use cmake to detect eigen (for example pagmo) typically use something like this:

find_package(Eigen3 3.3 REQUIRED CONFIG)

but with 5.x the major version compatility check rejects the installed 5.x versions

(although in some cases the code is still compatible)

the question is how to deal with this incompatibility in the client cmake code when the code is actually compatible with 3.x and 5.x ?

at least ./doc/TopicCMakeGuide.dox is outdated in that regard because it also uses 3.3 which does not work with v5

one quick hack is to drop the version specification (3.3 is old anyway):

find_package(Eigen3 REQUIRED CONFIG)

See also discussion in #2969 (closed)