Search for VexCL and create an imported target.

Unfortunately, we cannot us the imported targets. They add some compile
parameters using generator expressions based on the CXX_COMPILER_ID.
While we are using the system CXX compiler for most of the stuff, some
cuda code is compiled with nvcc which at least for some versions does
not support -Wno-catch-value (which gets passed as normal compiler
option).

opm/simulators/linalg/bda/amgclSolverBackend.hpp

@@ -43,6 +43,9 @@
 #include <amgcl/relaxation/cusparse_ilu0.hpp>
 #endif
 
+#if HAVE_VEXCL
+#include <amgcl/backend/vexcl.hpp>
+#endif
 namespace bda
 {