build: Add #ifdefs for requesting import as DLL vs static explicitly

Author: real-or-random

include/secp256k1.h

@@ -141,6 +141,14 @@ typedef int (*secp256k1_nonce_function)(
 #   define SECP256K1_API            __declspec (dllexport)
 #   define SECP256K1_API_VAR extern __declspec (dllexport)
 #  endif
+# elif defined(SECP256K1_IMPORT_AS_STATICLIB) && defined(SECP256K1_IMPORT_AS_DLL)
+#  error "At most one of SECP256K1_IMPORT_AS_STATICLIB and SECP256K1_IMPORT_AS_DLL must be defined."
+# elif defined(SECP256K1_IMPORT_AS_STATICLIB)
+#  define SECP256K1_API
+#  define SECP256K1_API_VAR  extern
+# elif defined(SECP256K1_IMPORT_AS_DLL)
+#  define SECP256K1_API             __declspec (dllimport)
+#  define SECP256K1_API_VAR  extern __declspec (dllimport)
 # elif defined _MSC_VER
 #  define SECP256K1_API
 #  define SECP256K1_API_VAR  extern __declspec (dllimport)