build: Add SECP256K1_API_VAR to fix importing variables from DLLs

This fixes a build issue with MSVC. While MSVC imports *functions*
from DLLs automatically when building a consumer of the DLL, it does
not import *variables* automatically. In these cases, we need an
explicit __declspec(dllimport).

This commit simply changes our logic to what the libtool manual
suggests, which has a very comprehensive writeup on the topic. Note
that in particular, this solution is carefully designed not to break
static linking. However, as described in the libtool manual,
statically linking the library with MSVC will output warning LNK4217.
This is still the best solution overall, because the warning is
merely a cosmetic issue.

include/secp256k1.h

@@ -145,21 +145,28 @@ typedef int (*secp256k1_nonce_function)(
 # define SECP256K1_NO_BUILD
 #endif
 
-/** At secp256k1 build-time DLL_EXPORT is defined when building objects destined
- *  for a shared library, but not for those intended for static libraries.
- */
-
-#ifndef SECP256K1_API
-# if defined(_WIN32)
-#  if defined(SECP256K1_BUILD) && defined(DLL_EXPORT)
-#   define SECP256K1_API __declspec(dllexport)
-#  else
-#   define SECP256K1_API
+/* Symbol visibility. See libtool manual, section "Windows DLLs". */
+#if defined(_WIN32) && !defined(__GNUC__)
+# ifdef SECP256K1_BUILD
+#  ifdef DLL_EXPORT
+#   define SECP256K1_API            __declspec (dllexport)
+#   define SECP256K1_API_VAR extern __declspec (dllexport)
 #  endif
-# elif defined(__GNUC__) && (__GNUC__ >= 4) && defined(SECP256K1_BUILD)
-#  define SECP256K1_API __attribute__ ((visibility ("default")))
+# elif defined _MSC_VER
+#  define SECP256K1_API
+#  define SECP256K1_API_VAR  extern __declspec (dllimport)
+# elif defined DLL_EXPORT
+#  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)
+#  define SECP256K1_API             __attribute__ ((visibility ("default")))
+#  define SECP256K1_API_VAR  extern __attribute__ ((visibility ("default")))
 # else
 #  define SECP256K1_API
+#  define SECP256K1_API_VAR  extern
 # endif
 #endif
 
@@ -231,10 +238,10 @@ typedef int (*secp256k1_nonce_function)(
  *
  *  It is highly recommended to call secp256k1_selftest before using this context.
  */
-SECP256K1_API extern const secp256k1_context *secp256k1_context_static;
+SECP256K1_API_VAR const secp256k1_context *secp256k1_context_static;
 
 /** Deprecated alias for secp256k1_context_static. */
-SECP256K1_API extern const secp256k1_context *secp256k1_context_no_precomp
+SECP256K1_API_VAR const secp256k1_context *secp256k1_context_no_precomp
 SECP256K1_DEPRECATED("Use secp256k1_context_static instead");
 
 /** Perform basic self tests (to be used in conjunction with secp256k1_context_static)
@@ -631,10 +638,10 @@ SECP256K1_API int secp256k1_ecdsa_signature_normalize(
  * If a data pointer is passed, it is assumed to be a pointer to 32 bytes of
  * extra entropy.
  */
-SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_rfc6979;
+SECP256K1_API_VAR const secp256k1_nonce_function secp256k1_nonce_function_rfc6979;
 
 /** A default safe nonce generation function (currently equal to secp256k1_nonce_function_rfc6979). */
-SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_default;
+SECP256K1_API_VAR const secp256k1_nonce_function secp256k1_nonce_function_default;
 
 /** Create an ECDSA signature.
  *

include/secp256k1_ecdh.h

@@ -27,11 +27,11 @@ typedef int (*secp256k1_ecdh_hash_function)(
 
 /** An implementation of SHA256 hash function that applies to compressed public key.
  * Populates the output parameter with 32 bytes. */
-SECP256K1_API extern const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256;
+SECP256K1_API_VAR const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256;
 
 /** A default ECDH hash function (currently equal to secp256k1_ecdh_hash_function_sha256).
  * Populates the output parameter with 32 bytes. */
-SECP256K1_API extern const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default;
+SECP256K1_API_VAR const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default;
 
 /** Compute an EC Diffie-Hellman secret in constant time
  *

include/secp256k1_schnorrsig.h

@@ -61,7 +61,7 @@ typedef int (*secp256k1_nonce_function_hardened)(
  *  Therefore, to create BIP-340 compliant signatures, algo must be set to
  *  "BIP0340/nonce" and algolen to 13.
  */
-SECP256K1_API extern const secp256k1_nonce_function_hardened secp256k1_nonce_function_bip340;
+SECP256K1_API_VAR const secp256k1_nonce_function_hardened secp256k1_nonce_function_bip340;
 
 /** Data structure that contains additional arguments for schnorrsig_sign_custom.
  *