make it easier to include rmm in other projects (#788)

Right now if we want to include RMM in other cmake projects using CPM, we'd have to do something like this:
```cmake
CPMAddPackage(
    NAME rmm
    GIT_REPOSITORY https://github.com/rapidsai/rmm
    GIT_TAG 0cc1380
    OPTIONS "BUILD_TESTS OFF" "BUILD_BENCHMARKS OFF"
)
```
With this change, it can be done using the short form:
```cmake
CPMAddPackage("gh:rapidsai/rmm#0cc1380523d31b2f044ecc74b457b228f8aea0c8")
```
This is also the recommendation from `Modern CMake` (https://cliutils.gitlab.io/modern-cmake/chapters/testing.html).

Authors:
  - Rong Ou (https://github.com/rongou)

Approvers:
  - Robert Maynard (https://github.com/robertmaynard)
  - Keith Kraus (https://github.com/kkraus14)

URL: #788

CMakeLists.txt

@@ -93,7 +93,7 @@ set_property(CACHE RMM_LOGGING_LEVEL PROPERTY STRINGS "TRACE" "DEBUG" "INFO" "WA
                                               "CRITICAL" "OFF")
 message(STATUS "RMM: RMM_LOGGING_LEVEL = '${RMM_LOGGING_LEVEL}'")
 
-if(BUILD_TESTS OR BUILD_BENCHMARKS)
+if((BUILD_TESTS OR BUILD_BENCHMARKS) AND CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
   # Auto-detect available GPU compute architectures
   include(${RMM_SOURCE_DIR}/cmake/Modules/SetGPUArchs.cmake)
   # Enable the CUDA language after setting CMAKE_CUDA_ARCHITECTURES
@@ -103,16 +103,16 @@ endif()
 
 # optionally build tests
 
-if(BUILD_TESTS)
+if(BUILD_TESTS AND CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
   enable_testing()
   add_subdirectory(tests)
-endif(BUILD_TESTS)
+endif()
 
 # add google benchmark
 
-if(BUILD_BENCHMARKS)
+if(BUILD_BENCHMARKS AND CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
   add_subdirectory(benchmarks)
-endif(BUILD_BENCHMARKS)
+endif()
 
 # install targets