Meson build: fix is_os_cocoa detection

The previous check didnt work as the compiler choked on the Objective-C code
that was dragged in by the Cocoa headers.

This patch simplifies things by simply asking what system the host is
running on.
1 job for is_os_cocoa-meson in 12 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #999484
library_build

00:12:55