Only use PR_SET_PTRACER if it is defined (e.g. on Ubuntu) - it's benign to be absent (e.g. on CentOS)
Merge branch 'dev' into pr_set_ptracer
Merge pull request #199 from Metaswitch/pr_set_ptracer

[Reviewer: Mike] Only use PR_SET_PTRACER if defined (not required otherwise)