gen_context: Don't use external ASM because it complicates the build

Fixes bitcoin#931.

Author: real-or-random

src/gen_context.c

@@ -13,6 +13,11 @@
 /* We can't require the precomputed tables when creating them. */
 #undef USE_ECMULT_STATIC_PRECOMPUTATION
 
+/* In principle we could use external ASM, but this yields only a minor speedup in
+   build time and it's very complicated. In particular when cross-compiling, we'd
+   need to build the external ASM for the build and the host machine. */
+#undef USE_EXTERNAL_ASM
+
 #include "include/secp256k1.h"
 #include "assumptions.h"
 #include "util.h"