find_package(RMM) can now be called multiple times safely (#811)

Previously it would try to add the `rmm::Thrust` target multiple times causing errors as a target with that name already existed.

Authors:
  - Robert Maynard (https://github.com/robertmaynard)

Approvers:
  - Paul Taylor (https://github.com/trxcllnt)
  - Mark Harris (https://github.com/harrism)

URL: #811

CMakeLists.txt

@@ -123,7 +123,9 @@ The goal of the [RMM](https://github.com/rapidsai/rmm) is to provide:
 
 set(code_string
     [=[
-thrust_create_target(rmm::Thrust FROM_OPTIONS)
+if(NOT TARGET rmm::Thrust)
+  thrust_create_target(rmm::Thrust FROM_OPTIONS)
+endif()
 ]=])
 
 rapids_export(