Check for __linux__ instead of linux during compilation

Check for __linux__ instead of linux during compilation

"__linux__" is the POSIX define for checking for Linux OS, "linux" is
deprecated and apparently not supported by Android.

Besides correcting the define, patch adds error for the case OS isn't
recognized.

Signed-off-by: Eero Tamminen <eero.t.tamminen@intel.com>
v2: Drop double negative from commit msg. Spotted by Mateusz Jończyk.
[Emil Velikov: Remove irrelevant Android details from commit msg.]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jasper St. Pierre <jstpierre@mecheye.net>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>