Fix the 'libpam required but missing' issue when cross-compile for ARM

The is a problem when using the openvpn-build to cross-compile for arm platform on Ubuntu 14.
after fired the command
IMAGEROOT=`pwd`/image-arm CHOST=arm-linux-gnueabi CBUILD=x86_64-pc-linux-gnu ./build

the error 'libpam required but missing' will shows up.
This is because there is NO libpam library in the cross-compile environment.

Considering the libpam is not common used(I suppose). I disabled it in the default options.

Cheers,
Billy YAO