proper use of the HAVE_TRACING variable defined by Cmake through -Dtracing=on

details:
- cleaning some code (instr/config.h) that was autotools-related
- created a Cmake rule to create a tracing_config.h that contains
the preprocessor variable HAVE_TRACING
- msg/datatypes.h and instr/instr.h needs to include tracing_config.h
to not segfault because of the size of the task struct
- .gitignore updated

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@7423 48e7efb5-ca39-0410-a469-dd3cf9ba447f