Untangle: replace manual int64 bodging with int64_t.

Where possible, that is. If our compilation environment has provided
int64_t, we can just make our int64 type be that, and not have to mess
around with multi-word arithmetic at all.

untangle.c

@@ -20,10 +20,6 @@
  *    requirements are adequately expressed by a single scalar tile
  *    size), and probably complicate the rest of the puzzles' API as a
  *    result. So I'm not sure I really want to do it.
- *
- *  - It would be nice if we could somehow auto-detect a real `long
- *    long' type on the host platform and use it in place of my
- *    hand-hacked int64s. It'd be faster and more reliable.
  */
 
 #include <stdio.h>
@@ -37,6 +33,9 @@
 #else
 #  include <tgmath.h>
 #endif
+#if HAVE_STDINT_H
+#  include <stdint.h>
+#endif
 
 #include "puzzles.h"
 #include "tree234.h"
@@ -221,6 +220,9 @@ static const char *validate_params(const game_params *params, bool full)
  * integer overflow at the very core of cross().
  */
 
+#if !HAVE_STDINT_H
+/* For prehistoric C implementations, do this the hard way */
+
 typedef struct {
     long hi;
     unsigned long lo;
@@ -300,6 +302,21 @@ static int64 dotprod64(long a, long b, long p, long q)
     return ab;
 }
 
+#else /* HAVE_STDINT_H */
+
+typedef int64_t int64;
+#define greater64(i,j) ((i) > (j))
+#define sign64(i) ((i) < 0 ? -1 : (i)==0 ? 0 : +1)
+#define mulu32to64(x,y) ((int64_t)(unsigned long)(x) * (unsigned long)(y))
+#define mul32to64(x,y) ((int64_t)(long)(x) * (long)(y))
+
+static int64 dotprod64(long a, long b, long p, long q)
+{
+    return (int64)a * b + (int64)p * q;
+}
+
+#endif /* HAVE_STDINT_H */
+
 /*
  * Determine whether the line segments between a1 and a2, and
  * between b1 and b2, intersect. We count it as an intersection if