config: Set preprocessor defaults for ECMULT_* config values

This simplifies manual builds and solves one item in bitcoin-core#929.

Author: real-or-random

src/ecmult.h

@@ -11,6 +11,9 @@
 #include "scalar.h"
 #include "scratch.h"
 
+#ifndef ECMULT_WINDOW_SIZE
+#  define ECMULT_WINDOW_SIZE 15
+#endif
 /* Noone will ever need more than a window size of 24. The code might
  * be correct for larger values of ECMULT_WINDOW_SIZE but this is not
  * tested.

src/ecmult_gen.h

@@ -10,6 +10,9 @@
 #include "scalar.h"
 #include "group.h"
 
+#ifndef ECMULT_GEN_PREC_BITS
+#  define ECMULT_GEN_PREC_BITS 4
+#endif
 #if ECMULT_GEN_PREC_BITS != 2 && ECMULT_GEN_PREC_BITS != 4 && ECMULT_GEN_PREC_BITS != 8
 #  error "Set ECMULT_GEN_PREC_BITS to 2, 4 or 8."
 #endif