include: Avoid visibility("default") on Windows

Fixes #1421.

include/secp256k1.h

@@ -147,6 +147,14 @@ typedef int (*secp256k1_nonce_function)(
      * 1. If using Libtool, it defines DLL_EXPORT automatically.
      * 2. In other cases, SECP256K1_DLL_EXPORT must be defined. */
 #   define SECP256K1_API extern __declspec (dllexport)
+#  else
+    /* Building libsecp256k1 as a static library on Windows.
+     * No declspec is needed. One may be tempted to set
+     * __attribute__ ((visibility("default"))), but this may trigger warnings on
+     * GCC when compiling with -flto, despite the GCC manual (Section "Common
+     * Function Attributes") claiming that default visibility is supported
+     * everywhere, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116478. */
+#   define SECP256K1_API extern
 #  endif
   /* The user must define SECP256K1_STATIC when consuming libsecp256k1 as a static
    * library on Windows. */