Check if libc has __xstat

uClibc doesn't use __xstat. Check if it exists, otherwise don't export
the override function.
diff --git a/configure.ac b/configure.ac
index f4bfe78..ce823d3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -30,6 +30,8 @@
 AC_PATH_PROG([XSLTPROC], [xsltproc])
 PKG_PROG_PKG_CONFIG
 
+AC_CHECK_FUNCS_ONCE(__xstat)
+
 AC_ARG_WITH([rootprefix],
         AS_HELP_STRING([--with-rootprefix=DIR], [rootfs directory prefix for config files and kernel modules]),
         [], [with_rootprefix=""])