Largefile fixup from Marcus.
diff --git a/configure.ac b/configure.ac
index 7c4d2f3..9bb9a00 100644
--- a/configure.ac
+++ b/configure.ac
@@ -91,7 +91,7 @@
 
 # Switches.
 # Enable LFS (Large File Support)
-CFLAGS="$CFLAGS -D_FILE_OFFSET_BITS=64"
+AC_SYS_LARGEFILE
 # Stick in "-Werror" if you want to be more aggressive.
 # (No need to use AC_SUBST on this default substituted environment variable.)
 CFLAGS="$CFLAGS -Wall -Wmissing-prototypes"