[[ VS2015 ]] Ensure libxlst builds under VS2015

This patch ensures libxlst builds using VS2015. It adds a
mapping for vsnprintf to the VS CRT equivalent.

Author: runrevmark

libxslt/include/libxslt/win32config.h

@@ -79,7 +79,6 @@ static int isnan (double d) {
 #include <direct.h>
 #if defined(_MSC_VER) || defined(__MINGW32__)
 #define mkdir(p,m) _mkdir(p)
-#define snprintf _snprintf
 #if _MSC_VER < 1500
 #define vsnprintf(b,c,f,a) _vsnprintf(b,c,f,a)
 #endif