Require use of GCC 4.4 or CLang compilers

Require use of GCC 4.4 or CLang compilers

We only ever test libvirt with GCC or CLang which provides a
GCC compatible compilation environment. Between them, these
compilers cover every important operating system platform,
even Windows.

Mandate their use to make it explicit that we don't care about
compilers like Microsoft VCC or other UNIX vendor C compilers.

GCC 4.4 was picked as the baseline, since RHEL-6 ships 4.4.7
and that lets us remove a large set of checks. There is a slight
issue that CLang reports itself as GCC 4.2, so we must also check
if __clang__ is defined. We could check a particular CLang version
too, but that would require someone to figure out a suitable min
version which is fun because OS-X reports totally different CLang
version numbers from CLang builds on Linux/BSD

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>