Move declarations of xmpp_{v,}snprintf.

Use HAVE_SNPRINTF and HAVE_VSNPRINTF to conditionnaly use an internal
version of these functions or use the libc one.

src/snprintf.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright Patrick Powell 1995
+ * This code is based on code written by Patrick Powell (papowell@astart.com)
+ * It may be used for any purpose as long as this notice remains intact
+ * on all source code distributions
+ */
+
+/** @file
+ *  Compatibility wrappers for OSes lacking snprintf(3) and/or vsnprintf(3).
+ */
+
+#ifndef __LIBSTROPHE_SNPRINTF_H__
+#define __LIBSTROPHE_SNPRINTF_H__
+
+#include <stddef.h>
+#include <stdarg.h>
+
+#if defined(HAVE_SNPRINTF) || defined(HAVE_VSNPRINTF)
+#include <stdio.h>
+#endif
+
+#ifdef HAVE_SNPRINTF
+#define xmpp_snprintf snprintf
+#else
+int xmpp_snprintf(char *str, size_t count, const char *fmt, ...);
+#endif
+
+#ifdef HAVE_VSNPRINTF
+#define xmpp_vsnprintf vsnprintf
+#else
+int xmpp_vsnprintf(char *str, size_t count, const char *fmt, va_list arg);
+#endif
+
+#endif /* __LIBSTROPHE_SNPRINTF_H__ */