patch 8.0.1681: the format attribute fails with MinGW

Problem:    The format attribute fails with MinGW. (John Marriott)
Solution:   Don't use the format attribute with MinGW.
diff --git a/src/proto.h b/src/proto.h
index 231c53f..655b72c 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -120,7 +120,7 @@
 _RTLENTRYF
 #  endif
 vim_snprintf_add(char *, size_t, char *, ...)
-#ifdef __GNUC__
+#ifdef USE_PRINTF_FORMAT_ATTRIBUTE
     __attribute__((format(printf, 3, 4)))
 #endif
     ;
@@ -130,7 +130,7 @@
 _RTLENTRYF
 #  endif
 vim_snprintf(char *, size_t, char *, ...)
-#ifdef __GNUC__
+#ifdef USE_PRINTF_FORMAT_ATTRIBUTE
     __attribute__((format(printf, 3, 4)))
 #endif
     ;
@@ -223,7 +223,7 @@
 
 /* Not generated automatically, to add extra attribute. */
 void ch_log(channel_T *ch, const char *fmt, ...)
-#ifdef __GNUC__
+#ifdef USE_PRINTF_FORMAT_ATTRIBUTE
     __attribute__((format(printf, 2, 3)))
 #endif
     ;