tests: use correct PID for main thread

Since PIDs are not beginning at 0 any more, this test was failing.
dist/tools: add check for extern "C" declaration
Travis: add externc check
Merge pull request #1832 from OlegHahm/test_thread_msg_seq_fixup

tests: use correct PID for main thread
Merge pull request #1789 from LudwigOrtmann/externc_check

CI: extern "C" check