TSQR: Only tentatively enable CU{BLAS,SOLVER} if CUDA enabled

Currently, there is no way to express that a TPL depends on another
TPL.  See discussion here:

trilinos#6496 (comment)

This means that we cannot use

TRIBITS_TPL_TENTATIVELY_ENABLE(CUBLAS)

(for example), unless we first protect it with IF(TPL_ENABLE_CUDA).
This commit does so for CUBLAS and CUSOLVER.

packages/tpetra/tsqr/cmake/Dependencies.cmake

@@ -7,5 +7,9 @@ SET(LIB_OPTIONAL_DEP_TPLS CUBLAS CUSOLVER)
 SET(TEST_REQUIRED_DEP_TPLS)
 SET(TEST_OPTIONAL_DEP_TPLS)
 
-TRIBITS_TPL_TENTATIVELY_ENABLE(CUBLAS)
-TRIBITS_TPL_TENTATIVELY_ENABLE(CUSOLVER)
+IF(DEFINED TPL_ENABLE_CUDA)
+  IF(TPL_ENABLE_CUDA)
+    TRIBITS_TPL_TENTATIVELY_ENABLE(CUBLAS)
+    TRIBITS_TPL_TENTATIVELY_ENABLE(CUSOLVER)
+  ENDIF()
+ENDIF()