Merge bitcoin#769: Undef HAVE___INT128 in basic-config.h to fix gen_context compilation

22e578b Undef HAVE___INT128 in basic-config.h to fix gen_context compilation (Tim Ruffing)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK 22e578b

Tree-SHA512: 91e11c3feade13923a01c30025b7f01d0cb6d7d88cd7a19d490373d2fb4552f2ca1ab0d9138096268999bcbfd51ef3c9af64ec8ab0dc8ee2fa60be16d2b5af64

Author: jonasnick

src/basic-config.h

@@ -25,6 +25,7 @@
 #undef USE_SCALAR_INV_BUILTIN
 #undef USE_SCALAR_INV_NUM
 #undef ECMULT_WINDOW_SIZE
+#undef HAVE___INT128 /* used in util.h */
 
 #define USE_NUM_NONE 1
 #define USE_FIELD_INV_BUILTIN 1