vcl/osx/sal: SDK 10.5 compatibility

NSWindowDelegate and NSMenuDelegate protocols, IOPMAssertion*
are available only in OS X v10.6 and later.

Change-Id: Icda4b8014d9a6dde2284b9f5df493eb470fb79fc
Reviewed-on: https://gerrit.libreoffice.org/8211
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>