Change how 64 bit builds are detected.

Instead of doing this at configure time, we look at the actual
compile time status. This also avoids hardcoding checks based on
what CPU architecture is present, which doesn't work when Z3 is
being built on non-x86_64 platforms.

src/util/mpn.h

@@ -61,7 +61,7 @@ public:
     char * to_string(mpn_digit const * a, size_t lng,
                      char * buf, size_t lbuf) const;
 private:
-    #ifdef _AMD64_
+    #if defined(__LP64__) || defined(_WIN64)
     class  mpn_sbuffer : public sbuffer<mpn_digit> {
     public:
         mpn_sbuffer() : sbuffer<mpn_digit>() {}