Allow Exclusion of `uninstall` target

When using cmake FetchContent to add Eigen as a dependency, I'm getting errors like:

Severity	Code	Description	Project	File	Line	Suppression State
Error		CMake Error at out/build/x64-Debug/_deps/eigen-src/CMakeLists.txt:615 (add_custom_target):
  add_custom_target cannot create target "uninstall" because another target
  with the same name already exists.  The existing target is a custom target
  created in source directory
  "C:/Users/user/project/out/build/x64-Debug/_deps/assimp-src".
  See documentation for policy CMP0002 for more details.	Eigen3	C:\Users\user/project\out/build/x64-Debug/_deps/eigen-src/CMakeLists.txt	615	

I can get it to work by fiddling with the order that the dependencies are built in, along with configs for those that let me turn targets on and off. Would be good if there was an option in Eigen as well to toggle off the uninstall target.