libext2fs: have UNIX IO manager use pread64/pwrite64

Commit baa3544609da3c ("libext2fs: have UNIX IO manager use
pread/pwrite) causes a breakage on 32-bit systems where off_t is
32-bits for file systems larger than 4GB.  Fix this by using
pread64/pwrite64 if possible, and if pread64/pwrite64 is not present,
using pread/pwrite only if the size of off_t is at least as big as
ext2_loff_t.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/configure.in b/configure.in
index b825f3d..2e59fde 100644
--- a/configure.in
+++ b/configure.in
@@ -1061,6 +1061,8 @@
 	prctl
 	pread
 	pwrite
+	pread64
+	pwrite64
 	secure_getenv
 	setmntent
 	setresgid