added check for git submodule checkout

[prateekchawla168]

Fixes #2777

Added code in CMakeLists.txt to check if the rajaperf submodule has been checked out.

If the compile-time flag -DKokkosKernels_ENABLE_TESTS_AND_PERFSUITE=ON is set and the submodule does not exist, the flag is switched off, and the user is informed to run git submodule update --init --recursive to download the submodule first. The compilation is allowed to proceed so the build does not fail.

[lucbv]

@prateekchawla168 the raja perf tests support was experimental and never got to a stable stage, we should really just remove it actually. Now we are using google perf test and CMake should pull that in automatically for you, we have two associated flags to build and run the tests.

[prateekchawla168]

I see, that makes sense. In that case it may be a good idea to remove this flag entirely, else the build still fails in case a user decides to think of using a single flag to activate both tests and the perf suite at the same time.