Make our replacement INT32_MAX always signed

The C standard says that INT32_MAX is supposed to be a signed
integer.  On platforms that have it, we get the correct
platform-defined value.  Our own replacement, however, was
unsigned.  That's going to cause a bug somewhere eventually.

src/common/torint.h

@@ -115,7 +115,7 @@ typedef unsigned int uint32_t;
 #define UINT32_MAX 0xffffffffu
 #endif
 #ifndef INT32_MAX
-#define INT32_MAX 0x7fffffffu
+#define INT32_MAX 0x7fffffff
 #endif
 #endif