Add travis build configuration

Add travis build configuration

This travis configuration tests libvirt builds on 5 platforms that we don't
exercise in the CentOS CI system.

  - Ubuntu Trusty with GCC
  - Ubuntu Trusty with CLang
  - Ubuntu Precise with GCC
  - Ubuntu Precise with CLang
  - OS-X with CLang

NB, syntax-check fails on OS-X with errors like:

   /bin/sh: /usr/bin/grep: Argument list too long

Presumably their grep impl isn't as good as the GNU one, so this test
config skips syntax-check on OS-X for now.

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