Fix handling of Linux systems without struct statfs64

* acinclude.m4 (AC_STATFS64): New macro to check for struct statfs64.
* configure.ac: Call AC_STATFS64.
* file.c (printstatfs64, sys_statfs64, sys_fstatfs64): Compile only
  if struct statfs64 is available.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
diff --git a/configure.ac b/configure.ac
index 96c1c24..9afee58 100644
--- a/configure.ac
+++ b/configure.ac
@@ -187,6 +187,7 @@
 		  struct stat.st_level,
 		  struct stat.st_rdev])
 AC_STAT64
+AC_STATFS64
 
 AC_TYPE_SIGNAL
 AC_TYPE_UID_T