fix make distcheck

Author: benma

Makefile.am

@@ -151,7 +151,7 @@ endif
 endif
 
 if USE_ECMULT_STATIC_PRECOMPUTATION
-CPPFLAGS_FOR_BUILD +=-I$(top_srcdir)
+CPPFLAGS_FOR_BUILD +=-I$(top_srcdir) -I$(builddir)/src
 
 gen_context_OBJECTS = gen_context.o
 gen_context_BIN = gen_context$(BUILD_EXEEXT)

src/gen_context.c

@@ -4,7 +4,11 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#include "libsecp256k1-config.h" // for ECMULT_GEN_PREC_BITS
+// Autotools creates libsecp256k1-config.h, of which ECMULT_GEN_PREC_BITS is needed.
+// ifndef guard so downstream users can define their own if they do not use autotools.
+#if !defined(ECMULT_GEN_PREC_BITS)
+#include "libsecp256k1-config.h"
+#endif
 #define USE_BASIC_CONFIG 1
 #include "basic-config.h"