find system blas as fallback when linking user code

AMDiS/AMDISConfig.cmake.in

@@ -121,7 +121,12 @@ if(AMDIS_NEED_UMFPACK)
 		list(APPEND AMDIS_LIBRARIES ${BLAS_LIBRARY})
 		list(APPEND AMDIS_LIBRARY_DIRS ${BLAS_LIBRARY_DIR})
 	  else()
-		message(ERROR "Could not find the BLAS library. Please set the variable BLAS_LIBRARY to the blas library with full path")
+		find_package(BLAS REQUIRED)
+		if(BLAS_FOUND)
+			list(APPEND AMDIS_LIBRARIES ${BLAS_LIBRARIES})
+		else()
+			message(FATAL_ERROR "Could not find the BLAS library. Please set the variable BLAS_LIBRARY to the blas library with full path")
+		endif()
 	  endif()
 	endif("$ENV{BLA_VENDOR}" STREQUAL "ACML")