make_manual: Support SOURCE_DATE_EPOCH
make_manual: Support SOURCE_DATE_EPOCH

The documentation contains an embedded datetime which do not read
SOURCE_DATE_EPOCH. This makes the documentation unreproducible when
distribution tries to recreate previous build of the package.

This patch ensures we attempt to read the environment variable before
default to the current build time.

Signed-off-by: Morten Linderud <d6326138d28a3329900796fdcf48fe9c3bddc3b1@linderud.pw>

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone