build: avoid exposing internal defines in the amalgamation code

This allows for easier inclusion of the source file directly in an
existing projects source, rather than compiling it as a separate file.

Author: jgriffiths

src/amalgamation/combined.c

@@ -38,6 +38,7 @@
  * config.h (which must be provided by the amalgamation user)
  * is included.
  */
+#define BUILD_AMALGAMATION 1
 #include "src/internal.h"
 
 /* The amalgamation user can provide their own defines and skip
@@ -121,3 +122,10 @@ void wally_silence_unused_warnings(void)
     assert_bip38_assumptions();
     assert_tx_assumptions();
 }
+
+/* Undefine our internal macros */
+#undef BYTES_VALID
+#undef BYTES_INVALID
+#undef BYTES_INVALID_N
+#undef OUTPUT_CHECK
+#undef OUTPUT_ALLOC

src/internal.h

@@ -73,13 +73,15 @@ bool mem_is_zero(const void *mem, size_t len);
 /* Fetch our internal operations function pointers */
 const struct wally_operations *wally_ops(void);
 
+#ifndef BUILD_AMALGAMATION
 #define malloc(size) __use_wally_malloc_internally__
 #define calloc(size) __use_wally_calloc_internally__
 #define free(ptr) __use_wally_free_internally__
 #ifdef strdup
 #undef strdup
 #endif
 #define strdup(ptr) __use_wally_strdup_internally__
+#endif
 
 #define NUM_ELEMS(a) (sizeof(a) / sizeof(a[0]))