Fix logfile destination for split compile/link
Report original error message in CompilationError
Add option to run compiler using os.system

On some systems (notably HPC machines) subprocess.check_call can result
in segfaults.  A somewhat safer option is to use os.system instead.  Set
the configuration option "no_fork_available" to True, or the environment
variable PYOP2_NO_FORK_AVAILABLE to 1 to enable this approach when
compiling code at runtime.
Merge pull request #409 from OP2/no-subprocess-call

* no-subprocess-call:
  Add option to run compiler using os.system
  Report original error message in CompilationError
  Fix logfile destination for split compile/link