Use __linux__ macro throughout, instead of ocasional __linux.

__linux is not universally defined across all standards versions, compilers and architectures. Specifically at C99 and up, it's not defined in GCC on powerpc platform.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28314

Bugzilla: http://llvm.org/bugs/show_bug.cgi?id=20380

Patch by Dimitri John Ledkov <dimitri.ledkov@canonical.com>