LU-4036 autoconf: don't check ext4 src when disable server

LU-4036 autoconf: don't check ext4 src when disable server

We should not check for ext4 src to determine if we want
to build server codes when user explicitly use --disable-server

Signed-off-by: Minh Diep <minh.diep@intel.com>
Change-Id: I04b45cf0c0837a7b8760b7ff46645cabfc2c6ebd
Reviewed-on: http://review.whamcloud.com/7818
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>