c99_alloca.h: add case for __sun

Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
diff --git a/include/c99_alloca.h b/include/c99_alloca.h
index 575f719..ed66fda 100644
--- a/include/c99_alloca.h
+++ b/include/c99_alloca.h
@@ -35,6 +35,10 @@
 
 #  define alloca _alloca
 
+#elif defined(__sun)
+
+#  include <alloca.h>
+
 #else /* !defined(_MSC_VER) */
 
 #  include <stdlib.h>