[Bug 215826] C++ program signal handlers not called

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215826

            Bug ID: 215826
           Summary: C++ program signal handlers not called
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: misc
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: kami at freebsd.org
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

18 days ago I got a bug report for sysutils/powerdxx:
https://github.com/lonkamikaze/powerdxx/issues/3

There is some distracting content in the report, e.g. the OP misidentified an
unrelated ACPI problem as part of the problem he/she was reporting.

The issue to sum it up is, that (at least my) C++ programs don't get their
signal handlers called (that includes default signal handlers that I never
touched).

Affected are the `loadrec` and `powerd++` binaries. The `loadrec` binary does
not touch signal handlers, so the default handlers stay in place.

I had a couple of people try to help me debug the issue and as far as we could
tell using truss and dtrace signals got sent, but the signal handlers just
don't get called. I don't know how to debug this further (how do you debug
something that is not happening?). But we found a workaround: link with
`-lpthread`. My assumption is that `pthread` replaces system functions with
thread safe versions and as a side effect replaces whatever got recently broken
on CURRENT.

I documented this workaround in the Makefile of my project:
https://github.com/lonkamikaze/powerdxx/commit/2d80d990121802b4402cf54bc9a328449ae8f326

I don't know when exactly it broke, I tested on head/r310361, the OP was
running head/r310173.

The problem occurs both if I include `signal(3)` via `<csignal>` or
`<signal.h>` or not at all (i.e. `loadrec`).

To reproduce get the last version without the workaround from the repo:
https://github.com/lonkamikaze/powerdxx/tree/93a755fbc4d7ec36e5a9d4a35d5a33052cc0e678

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