build: Simplify visibility logic

Author: real-or-random

include/secp256k1.h

@@ -174,18 +174,15 @@ typedef int (*secp256k1_nonce_function)(
 #  define SECP256K1_API             __declspec (dllimport)
 #  define SECP256K1_API_VAR  extern __declspec (dllimport)
 # endif
-#endif
-#ifndef SECP256K1_API
-# if defined(__GNUC__) && (__GNUC__ >= 4) && defined(SECP256K1_BUILD)
-   /* Building libsecp256k1 on non-Windows using GCC or compatible. */
-#  define SECP256K1_API             __attribute__ ((visibility ("default")))
-#  define SECP256K1_API_VAR  extern __attribute__ ((visibility ("default")))
-# else
+#elif defined(__GNUC__) && (__GNUC__ >= 4) && defined(SECP256K1_BUILD)
+  /* Building libsecp256k1 on non-Windows using GCC or compatible. */
+# define SECP256K1_API             __attribute__ ((visibility ("default")))
+# define SECP256K1_API_VAR  extern __attribute__ ((visibility ("default")))
+#else
    /* Linking against libsecp256k1,
       or building libsecp256k1 using an unknown compiler. */
-#  define SECP256K1_API
-#  define SECP256K1_API_VAR  extern
-# endif
+# define SECP256K1_API
+# define SECP256K1_API_VAR  extern
 #endif
 
 /* Warning attributes