Warn when functions are used on unsupported platforms.

[JohnLato]

As requested in #23 .

[dcoutts]

I think this is a good idea, but we should make sure the text of the warning explains what the behaviour is. So this is related to the other part of the discussion. For example if we make them throw proper IOErrors of type UnsupportedOperation, then we should make the warning say that, and it should also refer to the appropriate CPP defines if people really need to make compile time decisions on it.

For example, we might have:

"System.Posix.Files.setSymbolicLinkTImesHiRes: not available on this platform. It will throw an 'IOError' exception with error kind 'UnsupportedOperation'. Either catch this exception and provide your own fallback, or to decide at compile-time, use the CPP define HAVE_UTIMENSAT from \"HsUnix.h\"."

[hvr]

@dcoutts my only concern about that {-# WARNING #-} is how to generate it via CPP or similiar (as it's quite boilerplate-ish), and also how to break it in such a way that we don't have to go over 80 columns in the source-code.

PS: See also 98eced8 where I tried to do something in the spirit of #24 for new functions.

[hs-viktor]

Closing, rebased and reopened as #178