Resolve "use specific osx flags for pch"

What does this MR do?

Pass the correct macOS flags when compiling pch

How to test it?

Find a clean macOS 10.14 (Mojave) with no /usr/include directory, and launch cmake and ninja. Without the patch build should fail with a missing unistd.h

Data

(link to the needed data)

Some results

(Some interesting results, screenshot, perfs, ...)

Associated Issues/Merge Requests

• Issues
  • Fixes #211 (closed)