Merge remote-tracking branch 'origin/develop_integration_2018_w33' into develop

Summary of changes:
- fix compilation on CentOS
- some work on the T tracer:
  - add a 'multi' tracer to connect several tracers at the same time
  - improve 'to_vcd' to be compatible with previous VCD traces
- preliminary work for X2 handover
- various bug fixes

A new version of asn1c is to be used with this commit.
Do:
  cd [top directory of repository]
  source oaienv
  cd cmake_targets
  ./build_oai -I