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/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;