Fix static link failure against libssh2

configure.ac

@@ -489,6 +489,10 @@ if test "x$with_libssh2" = "xyes"; then
   PKG_CHECK_MODULES([LIBSSH2], [libssh2], [have_libssh2=yes], [have_libssh2=no])
   if test "x$have_libssh2" = "xyes"; then
     AC_DEFINE([HAVE_LIBSSH2], [1], [Define to 1 if you have libssh2.])
+
+    if test "x$ARIA2_STATIC" = "xyes"; then
+      LIBSSH2_CFLAGS="-DLIBSSH2_API= $LIBSSH2_CFLAGS"
+    fi
   else
     AC_MSG_WARN([$LIBSSH2_PKG_ERRORS])
     if test "x$with_libssh2_requested" = "xyes"; then