CMake: Restore conditional specification of BUILD_BYPRODUCTS when using an older CMake

This was removed because the previous eigen CMake spec forced a new CMake version
but that was taken out in a previous commit

cmake/external_eigen.cmake

@@ -21,7 +21,16 @@ set(EXTERNAL_INSTALL_LOCATION ${CMAKE_BINARY_DIR}/external)
 #----------------------------------------------------------------------------------------------------------
 
 # The 'BUILD_BYPRODUCTS' argument was introduced in CMake 3.2.
-ExternalProject_Add(
+if (${CMAKE_VERSION} VERSION_LESS 3.2)
+    ExternalProject_Add(
+        eigen
+        URL http://bitbucket.org/eigen/eigen/get/3.3.3.zip
+        # PREFIX ${CMAKE_CURRENT_BINARY_DIR}/eigen
+        UPDATE_COMMAND ""
+        CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTERNAL_INSTALL_LOCATION}
+        )
+else()
+    ExternalProject_Add(
         eigen
         URL http://bitbucket.org/eigen/eigen/get/3.3.3.zip
         # PREFIX ${CMAKE_CURRENT_BINARY_DIR}/eigen
@@ -29,6 +38,7 @@ ExternalProject_Add(
         CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTERNAL_INSTALL_LOCATION}
         BUILD_BYPRODUCTS "${EXTERNAL_INSTALL_LOCATION}/include/eigen3"
         )
+endif()
 
 #----------------------------------------------------------------------------------------------------------