tools/winebuild: fix a compiler warning on Cygwin

[Austin English]

-- 
-Austin
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diff --git a/include/wine/exception.h b/include/wine/exception.h
index 7e0d5bc..9277237 100644
--- a/include/wine/exception.h
+++ b/include/wine/exception.h
@@ -87,7 +87,13 @@ extern "C" {
 
 #if defined(__MINGW32__) || defined(__CYGWIN__)
 #define sigjmp_buf jmp_buf
+#ifdef sigsetjmp /*defined already on Cygwin, causes a conflict */
+#undef sigsetjmp
+#endif
 #define sigsetjmp(buf,sigs) setjmp(buf)
+#ifdef siglongjmp /*defined already on Cygwin, causes a conflict */
+#undef siglongjmp
+#endif
 #define siglongjmp(buf,val) longjmp(buf,val)
 #endif