Use always __linux__ for code that uses Linux-specific features

Use always __linux__ for code that uses Linux-specific features

See:
https://web.archive.org/web/20231206084513/http://www.faqs.org/docs/Linux-HOWTO/GCC-HOWTO.html#INDEX.25
"Use __linux__ for this purpose, not linux. Although the latter is
 defined, it is not POSIX compliant."

In fact, linux is not always defined:
TinyCC does not define linux, but __linux__.