Bump Eigen to versions 3.4.0 through 5.0.0

Eigen changed how their versioning works, so the new version is version 5.0.0. They still use Eigen3 as the name in cmake it seems.

Author: jkyang92

M2/cmake/check-libraries.cmake

@@ -127,7 +127,13 @@ find_package(GMP	6.0.0 REQUIRED)
 #   givaro	prime field and algebraic computations	(needs gmp)
 #  fflas_ffpack	Finite Field Linear Algebra Routines	(needs gmp, givaro + LAPACK)
 
-find_package(Eigen3	3.3.0 PATHS ${M2_HOST_PREFIX})
+find_package(Eigen3	3.4...5.0 PATHS ${M2_HOST_PREFIX})
+# FindEigen3 doesn't set EIGEN3_FOUND, and instead we should check
+# if the target Eigen3::Eigen is defined, so we set EIGEN3_FOUND
+# for compatibility with the rest of the build code
+if(TARGET Eigen3::Eigen)
+  set(EIGEN3_FOUND TRUE)
+endif (TARGET Eigen3::Eigen)
 find_package(BDWGC	7.6.4)
 find_package(MPFR	4.0.1)
 find_package(MPFI	1.5.1)