Check {GCC,CLANG}_VERSION==0 or >0 instead of using defined(), which
may render undefined behavior.

GCC_VERSION>0 implies defined(__GNUC__).

The ScopeExit library uses C++11 initializers, which gcc 4.6 does not
support.  Let's kill support for this ancient incomplete C++11
compiler, nobody should be using it anymore.

Copy the according file from another project (i.e. XCSoar).  This will
allow copying more code more easily.

We use range-based for and nullptr, which was not implemented prior to
gcc 4.6.

Remove GLib dependency from some headers.

Soon we'll use C++11 lambdas which were implemented in gcc 4.5.

This is specific to gcc, not to mpd.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Removed all macros which are already provided by GLib.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7167 09075e82-0dd4-0310-85a5-a0d7c8717e4f