Merge bitcoin-core/secp256k1#1306: build: Make tests work with external default callbacks

1907f0f build: Make tests work with external default callbacks (Tim Ruffing)

Pull request description:

ACKs for top commit:
  sipa:
    ACK 1907f0f
  jonasnick:
    ACK 1907f0f

Tree-SHA512: 198598f7bf5292bf5709187f9a40ddf9a0fba93e8b62afb49df2c05b4ef61c394cea43ee07615b51ceea97862228d8ad351fddef13c190cb2e6690943ed63128

Author: jonasnick

src/tests.c

@@ -10,7 +10,12 @@
 
 #include <time.h>
 
+#ifdef USE_EXTERNAL_DEFAULT_CALLBACKS
+    #pragma message("Ignoring USE_EXTERNAL_CALLBACKS in tests.")
+    #undef USE_EXTERNAL_DEFAULT_CALLBACKS
+#endif
 #include "secp256k1.c"
+
 #include "../include/secp256k1.h"
 #include "../include/secp256k1_preallocated.h"
 #include "testrand_impl.h"

src/tests_exhaustive.c

@@ -13,7 +13,12 @@
 #define EXHAUSTIVE_TEST_ORDER 13
 #endif
 
+#ifdef USE_EXTERNAL_DEFAULT_CALLBACKS
+    #pragma message("Ignoring USE_EXTERNAL_CALLBACKS in exhaustive_tests.")
+    #undef USE_EXTERNAL_DEFAULT_CALLBACKS
+#endif
 #include "secp256k1.c"
+
 #include "../include/secp256k1.h"
 #include "assumptions.h"
 #include "group.h"