support separate debug information

Introduce cpp.separateDebugInformation to toggle generation of separate
debug information files.
No OS X support for now.

Task-number: QBS-713
Change-Id: Id0f946376ffb3770bb179e638b34dc8a34b2d738
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
remove superfluous variables and assignments

Change-Id: I8545924b6577590a9e3f45e6b7c4ee89133a3a63
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
remove cpp.supportedStaticLibrarySuffixes

This internal property is not used anymore.

Change-Id: Iac27fc38d2e059fa39f48b424252a7c47091dad8
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>