replaced "#ifdef linux" with "#ifdef __linux"

The macro "__linux" is more reliable (Rolf Seuster reported that
the macro "linux" is not defined when using C++11).

From: Marco Clemencic <marco.clemencic@cern.ch>


git-svn-id: file:///afs/cern.ch/work/b/binet/dev/repos/gaudi-svn-mirror/Gaudi/trunk/GaudiKernel@7260 22b0f739-a4da-4a50-9948-337547c35a65