diff --git a/acinclude.m4 b/acinclude.m4
index c7fdc2f2d586d257ab7de23c5a4963506ce6f130..66cbdae45e417100aa91e41c7b007bf41e66c1a6 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -211,7 +211,7 @@ AC_DEFUN([CURL_FUNC_GETNAMEINFO_ARGTYPES], [
 #undef inline
 #ifdef HAVE_WINDOWS_H
 #define WIN32_LEAN_AND_MEAN
-#if defined(__MINGW32__) && (_WIN32_WINNT < 0x0501)
+#if defined(__MINGW32__) && ( (!defined(_WIN32_WINNT)) || (_WIN32_WINNT < 0x0501) )
 #undef _WIN32_WINNT
 #define _WIN32_WINNT 0x0501
 #endif
diff --git a/ares/acinclude.m4 b/ares/acinclude.m4
index 4f4e6fefc72bb0bf6c88b06ec87bf6a865b1f4e5..be1e4aed2414c41da85c1477f58d55fe3daddbf4 100644
--- a/ares/acinclude.m4
+++ b/ares/acinclude.m4
@@ -211,7 +211,7 @@ AC_DEFUN([CURL_FUNC_GETNAMEINFO_ARGTYPES], [
 #undef inline
 #ifdef HAVE_WINDOWS_H
 #define WIN32_LEAN_AND_MEAN
-#if defined(__MINGW32__) && (_WIN32_WINNT < 0x0501)
+#if defined(__MINGW32__) && ( (!defined(_WIN32_WINNT)) || (_WIN32_WINNT < 0x0501) )
 #undef _WIN32_WINNT
 #define _WIN32_WINNT 0x0501
 #endif