Build: Require Automake 1.12 and use serial-tests option.

It should actually still work with Automake 1.10 if
the serial-tests option is removed. Automake 1.13 started
using parallel tests by default and the option to get
the old behavior isn't supported before 1.12.

At least for now, parallel tests don't improve anything
in XZ Utils but they hide the progress output from
test_compress.sh.
1 file changed