RFR: 8351323: Parameterize compiler and linker flags for iconv

[snake66]

Allows for future support for platforms that require different flags for libiconv support.

Sponsored-by: The FreeBSD Foundation

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Commit messages:
 - 8351323: Parameterize libiconv compiler and linker flags

Changes: https://git.openjdk.org/jdk/pull/23995/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23995&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351323
  Stats: 37 lines in 5 files changed: 31 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/23995.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23995/head:pull/23995

PR: https://git.openjdk.org/jdk/pull/23995