Use empty string instead of zero to indicate not found

When writing the variable HAVE_SUPERLU to the config.h file, an empty
string will be interpreted as "not defined". Thus, we can use both
if and ifdef preprocessor directives to check for it. If we use zero,
then we must be careful to always use if, never ifdef.

cmake/Modules/FindSuperLU.cmake

@@ -159,5 +159,5 @@ endif(SUPERLU_FOUND)
 if(SUPERLU_FOUND)
   set(HAVE_SUPERLU 1)
 else(SUPERLU_FOUND)
-  set(HAVE_SUPERLU 0)
+  set(HAVE_SUPERLU "")
 endif(SUPERLU_FOUND)