ushare: do not use the DISABLE_NLS variable

ushare is not an autoconf/automake based package, so it does not have
the standard behavior of autoconf that every enable/disable option
supports both --enable-foo and --disable-foo.

For example, it supports --disable-nls, but not
--enable-nls. Currently, DISABLE_NLS is either empty or --disable-nls,
but this will be changed in a follow-up commit. As a preparation, this
commit modifies ushare to no longer use DISABLE_NLS, but instead
calculate itself whether --disable-nls should be passed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>