[cmake] avoid guard redefinitions

eigen and tbb might've already be discovered by pdelab/common

config.h.cmake

@@ -17,10 +17,14 @@
 #define DUNE_STUFF_VERSION_REVISION ${DUNE_STUFF_VERSION_REVISION}
 
 /* Define to 1 if eigen was found, else 0 */
+#ifndef HAVE_EIGEN
 #define HAVE_EIGEN ${HAVE_EIGEN}
+#endif
 
 /* Define to 1 if threading building blocks were found, else 0 */
+#ifndef HAVE_TBB
 #define HAVE_TBB ${HAVE_TBB}
+#endif
 
 #cmakedefine HAS_WORKING_UNUSED_ATTRIBUTE  1