Build naemon-livestatus on non-GNU environment

[dereckson]

To be able to build naemon-livestatus everywhere, there are some blockers:

• A forgotten header, see Include explicitly pthread headers #118
• Remove register keywords deprecated in C++11, removed from C++17, see Keyword register has been deprecated in C++ 11 and removed in C++17 #119
• Handle GNU-only glibc pthread_tryjoin_np

The last one is less trivial: 1bd7a3b uses an addition to pthreads functions only offered in glibc, using the function pthread_tryjoin_np missing from standard pthreads, that fonction uses glibc-only __pthread_clockjoin_ex etc.

It could be possible to include them if glibc is missing, as licenses of glibc and naemon-livestatus are compatible, but that's a lot of code, could someone has a link to that MON-9123 issue to look at the original issue this code change wanted to fix? It seems a JIRA one?

[dereckson]

Created #121 to protect patch introducing pthread_tryjoin_np with __ifdef__ GLIBC.