Fix compile in OS X

[Yuë Liu]

Hello,

When I was packaging Vc on OSX, I found there is a macro detection
problem. Clang will define __GNUC__, but libc++ doesn't contain
ext/stdio_sync_filebuf.h, so in include/Vc/IO it will fail compile, to
fix it, just replace all the
#ifdef __GNUC__
in that file to
#if defined __GNUC__ && ! defined __APPLE__

Also, I've made a mac homebrew formula for Vc on github, on your
website you can tell osx users to use that.

howto:

1. install homebrew on osx

2. add calligra tap

brew tap philacs/homebrew-calligra

3. install Vc

brew install vc

Cheers,
Yue Liu