configure.ac: Add AC_SYS_LARGEFILE for mmap()ing dumb GEM buffers

We're mmap()ing buffers with 64 bit offsets coming from the
DRM_IOCTL_MODE_MAP_DUMB ioctl and need to make sure mmap accepts
those.
diff --git a/configure.ac b/configure.ac
index e305ab0..156237f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -19,6 +19,7 @@
 AC_CONFIG_HEADERS([config.h])
 
 AC_USE_SYSTEM_EXTENSIONS
+AC_SYS_LARGEFILE
 
 AM_INIT_AUTOMAKE([1.11 parallel-tests foreign no-dist-gzip dist-xz color-tests])