build: test for __declspec(dllexport) in configure

This should work for GCC and Clang when building for Windows targets.

configure.ac

@@ -813,7 +813,6 @@ if test x$ac_cv_sys_large_files != x &&
   CPPFLAGS="$CPPFLAGS -D_LARGE_FILES=$ac_cv_sys_large_files"
 fi
 
-AX_GCC_FUNC_ATTRIBUTE([dllexport])
 AX_GCC_FUNC_ATTRIBUTE([dllimport])
 
 if test x$use_glibc_compat != xno; then
@@ -1000,6 +999,18 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([
   ]
 )
 
+AC_MSG_CHECKING([for dllexport attribute])
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+  __declspec(dllexport) int foo(void);
+  int main(){}
+  ])],
+  [
+    AC_DEFINE(HAVE_DLLEXPORT_ATTRIBUTE,1,[Define if the dllexport attribute is supported.])
+    AC_MSG_RESULT(yes)
+  ],
+  [AC_MSG_RESULT(no)]
+)
+
 dnl thread_local is currently disabled when building with glibc back compat.
 dnl Our minimum supported glibc is 2.17, however support for thread_local
 dnl did not arrive in glibc until 2.18.

src/script/bitcoinconsensus.h

@@ -11,12 +11,10 @@
 #if defined(BUILD_BITCOIN_INTERNAL) && defined(HAVE_CONFIG_H)
 #include <config/bitcoin-config.h>
   #if defined(_WIN32)
-    #if defined(DLL_EXPORT)
-      #if defined(HAVE_FUNC_ATTRIBUTE_DLLEXPORT)
-        #define EXPORT_SYMBOL __declspec(dllexport)
-      #else
-        #define EXPORT_SYMBOL
-      #endif
+    #if defined(HAVE_DLLEXPORT_ATTRIBUTE)
+      #define EXPORT_SYMBOL __declspec(dllexport)
+    #else
+      #define EXPORT_SYMBOL
     #endif
   #elif defined(HAVE_DEFAULT_VISIBILITY_ATTRIBUTE)
     #define EXPORT_SYMBOL __attribute__ ((visibility ("default")))