RFR: 8290466: Default to --with-source-date=current to avoid unmodified Hotspot recompilation

[Aleksey Shipilev]

See the bug for symptoms and rationale.

In short, after [JDK-8288396](https://bugs.openjdk.org/browse/JDK-8288396) moved the timestamp to `CFLAGS_VM_VERSION` macro, which changes with every build, we get Hotspot recompilation every time, even when Hotspot is not modified.

I believe current behavior is correct for `--with-source-date=updated` (current default): you do Hotspot build, you get updated build timestamp, which forces recompilation of affected compilation units, relinkage, re-creation of jmods/jimage. The way out is to switch to `--with-source-date=current`, which would only get the build timestamp at configure time.

CIs and build farms are likely reconfiguring before every build anyway, so their behavior is unlikely to be affected. But this switch definitely improves the day-to-day OpenJDK development work: it saves >50 seconds on every `make` for my machine, see below.

Back-to-back unmodified build before:


$ CONF=linux-x86_64-server-release time make hotspot
Building target 'hotspot' in configuration 'linux-x86_64-server-release'
Finished building target 'hotspot' in configuration 'linux-x86_64-server-release'

39.03user 10.97system 0:56.89elapsed 87%CPU (0avgtext+0avgdata 2048944maxresident)k
363616inputs+4108072outputs (484major+1100154minor)pagefaults 0swaps

$ CONF=linux-x86_64-server-release time make hotspot
Building target 'hotspot' in configuration 'linux-x86_64-server-release'
Finished building target 'hotspot' in configuration 'linux-x86_64-server-release'

38.18user 9.02system 0:49.18elapsed 95%CPU (0avgtext+0avgdata 2048788maxresident)k
40inputs+4101112outputs (0major+1100077minor)pagefaults 0swaps


Back-to-back unmodified build after:


$ CONF=linux-x86_64-server-release time make hotspot
Building target 'hotspot' in configuration 'linux-x86_64-server-release'
Finished building target 'hotspot' in configuration 'linux-x86_64-server-release'

2.15user 0.22system 0:02.59elapsed 91%CPU (0avgtext+0avgdata 49480maxresident)k
0inputs+176outputs (0major+55506minor)pagefaults 0swaps

$ CONF=linux-x86_64-server-release time make hotspot
Building target 'hotspot' in configuration 'linux-x86_64-server-release'
Finished building target 'hotspot' in configuration 'linux-x86_64-server-release'

2.18user 0.14system 0:02.46elapsed 94%CPU (0avgtext+0avgdata 49332maxresident)k

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/9638/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9638&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290466
  Stats: 8 lines in 3 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/9638.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9638/head:pull/9638

PR: https://git.openjdk.org/jdk/pull/9638