[CMake] Detect the operating system in a more generic way.

[CMake] Detect the operating system in a more generic way.
https://bugs.webkit.org/show_bug.cgi?id=67482

Patch by Raphael Kubo da Costa <kubo@profusion.mobi> on 2011-09-26
Reviewed by Adam Barth.

So far the buildsystem only considered Windows, Linux and Mac OS X as
valid operating systems, but any Unix (or at least the BSDs) should also
work fine.

By using the OS values CMake itself defines we can check for Unix
systems in a more generic fashion.

* Source/CMakeLists.txt:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@95986 268f45cc-cd09-0410-ab3c-d52691b4dbfc