Update logic for TPL_ENABLE_Kokkos=ON (trilinos#11545)

When Kokkos is supplied as an external package, the modern CMake imported
targets from the Kokkos<Subpkg>Config.cmake files also provide the needed
flags.  Therefore, there should be no special mention of Kokkos in the
Trilinos configure logic.

cmake/ProjectCompilerPostConfig.cmake

@@ -1,4 +1,6 @@
-IF (${PROJECT_NAME}_ENABLE_Kokkos)
+tribits_get_package_enable_status(Kokkos  KokkosEnable "")
+
+IF (KokkosEnable)
   MESSAGE("-- " "Skip adding flags for OpenMP because Kokkos flags does that ...")
   SET(OpenMP_CXX_FLAGS_OVERRIDE " ")
 
@@ -7,4 +9,6 @@ IF (${PROJECT_NAME}_ENABLE_Kokkos)
   # Grab those variables here so we know what was the original top-level flags
   # and what are the CMAKE_CXX_FLAGS added afterwards for an individual package
   SET(TRILINOS_TOPLEVEL_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+  # NOTE: Setting TRILINOS_TOPLEVEL_CXX_FLAGS only has any impact if Kokkos is
+  # being treated as an internal package.
 ENDIF()