Add flags to opt out from the instrumentation

[AnnaAr321]

The library work wonderfully (thanks!). In the commit, I added flags to opt out of tracking when I know it's not required.

an example of how to opt out from everything:

export INSTRUMENT_SLEEP=0
export INSTRUMENT_CONDVAR=0
export INSTRUMENT_MUTEX=0
export INSTRUMENT_SEMAPHORE=0
export INSTRUMENT_MISC=0

[dsharlet]

Thanks for the PR. Sorry about the delay, I'm apparently not getting notified for things on this repo. I'll fix that.

The overall idea of this change makes sense, I have a few minor suggestions about the implementation.

[dsharlet]

I'm a little worried about the global constructor to initialize the flags. I'm wondering, would the following pattern work?

if (!hooks::sleep) g_instrument_sleep = hooks::init(hooks::sleep, __sleep, "sleep", "INSTRUMENT_SLEEP");
if (!g_instrument_sleep) {
  return hooks::sleep(secs);
}

• Add a parameter to hooks::init that takes a string to indicate which environment variable should guard that function.
• Return the value of the flag that controls instrumentation of that function.

Mostly it's the same as your change, just initializing the global flags differently to avoid potential global constructor ordering issues.

It assigns the same flag multiple times, which is a bit of a wart, but only a few times (once per function).

[dsharlet]

Should these environment variables be prefixed with PTHREAD_TRACE_, to avoid global name collisions? Maybe PTHREAD_TRACE_SLEEP, PTHREAD_TRACE_CONDVAR, etc.?