c99_alloca.h: Also use <alloca.h> for cygwin

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Brian Paul <brianp@vmware.com>
diff --git a/include/c99_alloca.h b/include/c99_alloca.h
index ed66fda..5a3b8c1 100644
--- a/include/c99_alloca.h
+++ b/include/c99_alloca.h
@@ -35,7 +35,7 @@
 
 #  define alloca _alloca
 
-#elif defined(__sun)
+#elif defined(__sun) || defined(__CYGWIN__)
 
 #  include <alloca.h>