amalgamation: include ccan_config.h explicitly, protect from multiple inclusion

With this change, users of the amalgamation only need to provide
wally-specific defines in their config.h.

Author: jgriffiths

src/amalgamation/combined.c

@@ -33,7 +33,19 @@
 #include "src/secp256k1/src/secp256k1.c"
 #include "src/secp256k1/src/precomputed_ecmult_gen.c"
 #include "src/secp256k1/src/precomputed_ecmult.c"
-#include "src/ccan/ccan/crypto/sha256/sha256.c"
+
+/* Force the inclusion of our internal header first, so that
+ * config.h (which must be provided by the amalgamation user)
+ * is included.
+ */
+#include "src/internal.h"
+
+/* The amalgamation user can provide their own defines and skip
+ * providing a ccan_config.h if they define _WALLY_CCAN_CONFIG_H_.
+ */
+#ifndef _WALLY_CCAN_CONFIG_H_
+#include "src/ccan_config.h"
+#endif
 
 #include "src/internal.c"
 #include "src/address.c"
@@ -66,6 +78,9 @@
 #include "src/wif.c"
 #include "src/wordlist.c"
 
+/* ccan sources */
+#include "src/ccan/ccan/crypto/sha256/sha256.c"
+
 /* Redefine internal names so sha-512 can be included without conflicts */
 #define Round Round_512
 #define Transform Transform_512

src/ccan_config.h

@@ -1,4 +1,8 @@
+#ifndef _WALLY_CCAN_CONFIG_H_
+#define _WALLY_CCAN_CONFIG_H_ 1
+
 /* Config directives for ccan */
+
 #include <stddef.h>
 
 #ifdef WORDS_BIGENDIAN
@@ -55,3 +59,5 @@
 void wally_clear(void *p, size_t len);
 
 #define CCAN_CLEAR_MEMORY(p, len) wally_clear(p, len)
+
+#endif /*_WALLY_CCAN_CONFIG_H_ */