From a1634aa8d3d41a257ad659788dac222739df89f5 Mon Sep 17 00:00:00 2001
From: Demi Obenour <demiobenour@gmail.com>
Date: Tue, 2 May 2017 20:05:14 -0400
Subject: Honor user definitions of yy_* macros

The user may have defined the yy_* macros themselves.  In that case,
don't clobber them.
---
 src/flex.skl | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/flex.skl b/src/flex.skl
index e125e3a..d9854f4 100644
--- a/src/flex.skl
+++ b/src/flex.skl
@@ -112,9 +112,15 @@ m4_ifdef( [[M4_YY_REENTRANT]],  [[m4_define([[M4_YY_HAS_START_STACK_VARS]])]])
 m4_ifdef( [[M4_YY_PREFIX]],, [[m4_define([[M4_YY_PREFIX]], [[yy]])]])
 
 m4preproc_define(`M4_GEN_PREFIX',``
-[[#define yy$1 ]]M4_YY_PREFIX[[$1]]
+[[#ifdef yy$1
+#define ]]M4_YY_PREFIX[[$1_ALREADY_DEFINED
+#else
+#define yy$1 ]]M4_YY_PREFIX[[$1
+#endif]]
 'm4preproc_divert(1)`
-[[#undef yy$1]]'m4preproc_divert(0)')
+[[#ifndef ]]M4_YY_PREFIX[[$1_ALREADY_DEFINED
+#undef yy$1
+#endif]]'m4preproc_divert(0)')
 
 %if-c++-only
     /* The c++ scanner is a mess. The FlexLexer.h header file relies on the
-- 
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