[jdk15u-dev] RFR: 8261109: [macOS] Remove disabled warning for JNF in make/autoconf/flags-cflags.m4

[Vladimir Kempik]

Clean backport to jdk15u-dev. macos-only change to build system.

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Commit messages:
 - Backport 3bb6a3d2ab96373baa4e18f4e61173f11225fd4e

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/80/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=80&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261109
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/80.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/80/head:pull/80

PR: https://git.openjdk.java.net/jdk15u-dev/pull/80