Supporting GCC 4.9 is becoming cumbersome as we refactor more code to
C++14.

This shouldn't be a problem for anybody, because Debian Stable
(Stretch) has version 6.3, and Ubuntu LTS (Xenial) has version 5.3.

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.