RFR: 8344559: Log is spammed by missing pandoc warnings when building man pages

[Magnus Ihse Bursie]

If the user does not have pandoc installed and configured, the logs are spammed with the warning:


Warning: pandoc not found. Not generating man pages


once for each man page that should have been generated. 

This is excessive and should be removed. Instead, let configure inform the user that you cannot build man pages without pandoc.

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Commit messages:
 - 8344559: Log is spammed by missing pandoc warnings when building man pages

Changes: https://git.openjdk.org/jdk/pull/22573/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22573&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344559
  Stats: 13 lines in 4 files changed: 0 ins; 2 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/22573.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22573/head:pull/22573

PR: https://git.openjdk.org/jdk/pull/22573