build: Introduce SECP256k1_DLL_EXPORT macro

This change provides a way to build a shared library that is not tired
to the Libtool-specific `DLL_EXPORT` macro.

include/secp256k1.h

@@ -134,10 +134,12 @@ typedef int (*secp256k1_nonce_function)(
 #endif
 
 /* Symbol visibility. See https://gcc.gnu.org/wiki/Visibility */
-/* DLL_EXPORT is defined internally for shared builds */
 #if defined(_WIN32)
-# ifdef SECP256K1_BUILD
-#  ifdef DLL_EXPORT
+# if defined(SECP256K1_BUILD)
+#  if defined(DLL_EXPORT) || defined(SECP256K1_DLL_EXPORT)
+    /* Building libsecp256k1 as a DLL.
+     * 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)
 #  endif
   /* The user must define SECP256K1_STATIC when consuming libsecp256k1 as a static

src/CMakeLists.txt

@@ -20,10 +20,9 @@ if(SECP256K1_ASM STREQUAL "arm32")
   target_link_libraries(secp256k1_asm INTERFACE secp256k1_asm_arm)
 endif()
 
-# Define our export symbol only for Win32 and only for shared libs.
-# This matches libtool's usage of DLL_EXPORT
 if(WIN32)
-  set_target_properties(secp256k1 PROPERTIES DEFINE_SYMBOL "DLL_EXPORT")
+  # Define our export symbol only for shared libs.
+  set_target_properties(secp256k1 PROPERTIES DEFINE_SYMBOL SECP256K1_DLL_EXPORT)
   target_compile_definitions(secp256k1 INTERFACE $<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:SECP256K1_STATIC>)
 endif()