[jdk8u-dev] RFR: 8374948: [8u] saproc & jsig builds add duplicate linker flags on Darwin/MacOS

[Andrew John Hughes]

This patch breaks out the Mac minimum version argument into its own variable, `MAC_FLAGS`, in `gcc.make`, so it can be added independently in `saproc.make` and `jsig.make`. This prevents duplicate linker flags and C++-only flags being added to the C build for jsig and saproc.

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/740

Commit messages:
 - 8374948: [8u] saproc & jsig builds add duplicate linker flags on Darwin/MacOS

Changes: https://git.openjdk.org/jdk8u-dev/pull/742/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=742&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374948
  Stats: 9 lines in 3 files changed: 2 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/742.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/742/head:pull/742

PR: https://git.openjdk.org/jdk8u-dev/pull/742