`__linux` is the deprecated non-standard macros which appears to be
not present at all on PowerPC.

Closes #398

Fixes the NFS client build, closes #305.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

It does not do much harm - but it causes an unnecessary (failing)
system call.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.