Reorganize CMake and minimize configuration for non-top-level builds.

cmake/EigenTesting.cmake

@@ -91,6 +91,7 @@ macro(ei_add_test_internal testname testname_with_suffix)
   if(EIGEN_TEST_CUSTOM_LINKER_FLAGS)
     target_link_libraries(${targetname} ${EIGEN_TEST_CUSTOM_LINKER_FLAGS})
   endif()
+  target_link_libraries(${targetname} Eigen3::Eigen)
 
   if(${ARGC} GREATER 3)
     set(libs_to_link ${ARGV3})

lapack/CMakeLists.txt

@@ -110,6 +110,7 @@ foreach(target IN LISTS EIGEN_LAPACK_TARGETS)
   if(EIGEN_STANDARD_LIBRARIES_TO_LINK_TO)
     target_link_libraries(${target} ${EIGEN_STANDARD_LIBRARIES_TO_LINK_TO})
   endif()
+  target_link_libraries(${target} Eigen3::Eigen)
   add_dependencies(lapack ${target})
   install(TARGETS ${target}
           RUNTIME DESTINATION bin

unsupported/CMakeLists.txt

@@ -4,7 +4,7 @@ if(EIGEN_BUILD_DOC)
 endif()
 if(EIGEN_BUILD_TESTING)
   if(EIGEN_LEAVE_TEST_IN_ALL_TARGET)
-    add_subdirectory(test) # can't do EXCLUDE_FROM_ALL here, breaks CTest
+    add_subdirectory(test) # CTest automatic test building relies on the "all" target.
   else()
     add_subdirectory(test EXCLUDE_FROM_ALL)
   endif()