Define SECP256K1_BUILD in secp256k1.c directly.

This avoids building without it and makes it safer to use a custom
 building environment.  Test harnesses need to #include secp256k1.c
 first now.

src/bench_ecmult.c

@@ -5,8 +5,9 @@
  ***********************************************************************/
 #include <stdio.h>
 
-#include "include/secp256k1.h"
+#include "secp256k1.c"
 
+#include "include/secp256k1.h"
 #include "util.h"
 #include "hash_impl.h"
 #include "field_impl.h"
@@ -14,7 +15,6 @@
 #include "scalar_impl.h"
 #include "ecmult_impl.h"
 #include "bench.h"
-#include "secp256k1.c"
 
 #define POINTS 32768
 

src/bench_internal.c

@@ -5,8 +5,9 @@
  ***********************************************************************/
 #include <stdio.h>
 
-#include "include/secp256k1.h"
+#include "secp256k1.c"
 
+#include "include/secp256k1.h"
 #include "assumptions.h"
 #include "util.h"
 #include "hash_impl.h"
@@ -16,7 +17,6 @@
 #include "ecmult_const_impl.h"
 #include "ecmult_impl.h"
 #include "bench.h"
-#include "secp256k1.c"
 
 typedef struct {
     secp256k1_scalar scalar[2];

src/secp256k1.c

@@ -4,6 +4,8 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
+#define SECP256K1_BUILD
+
 #include "include/secp256k1.h"
 #include "include/secp256k1_preallocated.h"
 
@@ -21,6 +23,10 @@
 #include "scratch_impl.h"
 #include "selftest.h"
 
+#ifdef SECP256K1_NO_BUILD
+# error "secp256k1.h processed without SECP256K1_BUILD defined while building secp256k1.c"
+#endif
+
 #if defined(VALGRIND)
 # include <valgrind/memcheck.h>
 #endif

src/tests_exhaustive.c

@@ -20,10 +20,10 @@
 #define EXHAUSTIVE_TEST_ORDER 13
 #endif
 
+#include "secp256k1.c"
 #include "include/secp256k1.h"
 #include "assumptions.h"
 #include "group.h"
-#include "secp256k1.c"
 #include "testrand_impl.h"
 
 static int count = 2;