A placeholder to handle GNU make jobserver option.
spec/default.mspec didn't handle the jobserver using a FIFO.

This reverts commit 8986115e0a2a989f2b2ea5945f02c7a13989d640.
`RELEASE_DATE` including `YEAR`, `MONTH`, `DAY` are mandatory, while
`REVISION` is not.

The regexp used in EnvUtil.current_parser did not allow square brackets
for feature names. Modular GC uses square brackets for the GC name (such
as +GC[mmtk]).

test-unit depends on power_assert that the recent versions require
ruby 3.1 or later.  While test-unit rescues syntax error at loading
power_assert for old ruby versions, warnings for experimental features
are not suppressed, and `assert_separately` fails because stderr is
not empty, by default.  Since adding `required_ruby_version` to
power_assert causes the installation with old rubygems to fail, we
just ignore warnings for experimental features totally.