[libxml2] Fix compilation with Clang

[Daniel Veillard <veillard src gnome org>]

commit e306260f543aa4c47e8087b2eb013bb7586d801f
Author: Koop Mast <kwm freebsd org>
Date:   Wed Nov 3 16:10:26 2010 +0100

    Fix compilation with Clang
    
    Don't redefine vsnprintf if it is available

 python/libxml.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/python/libxml.c b/python/libxml.c
index 3f01761..b0043e6 100644
--- a/python/libxml.c
+++ b/python/libxml.c
@@ -30,7 +30,7 @@
 
 #if (defined(_MSC_VER) || defined(__MINGW32__)) && !defined(vsnprintf)
 #define vsnprintf(b,c,f,a) _vsnprintf(b,c,f,a)
-#elif defined(WITH_TRIO)
+#elif defined(WITH_TRIO) && !defined(vsnprintf)
 #include "trio.h"
 #define vsnprintf trio_vsnprintf
 #endif