In static builds find_package(Eigen3) is skipped, leaving Eigen3::Eigen
undefined when admesh and clipper try to link against it. Add an
unconditional find_package(Eigen3 CONFIG QUIET) guard in the main
CMakeLists before add_subdirectory(deps_src). deps_src/eigen now simply
provides the unnamespaced alias and fatals clearly if the target is missing.