change macOS < 10.12 clock to SYSTEM_CLOCK, fixes #2537

[Asmod4n]

This should fix the issue in #2537 and also make it work with leap seconds

[Asmod4n]

Have found the SYSTEM_CLOCK definition way back in code from the year 2000, so it should work everywhere.

[bluca]

Fails to build on the Travis OSX runs:

../../src/clock.cpp:64:21: error: use of undeclared identifier 'CLOCK_MONOTONIC'
    if (clock_id != CLOCK_MONOTONIC) {

Also could you please rebase away the merge commit?

[Asmod4n]

Strange, thought the #ifdef for CLOCK_MONOTONIC the line 154 should work, got a fix coming.

[Asmod4n]

Doesn't the merge button have that option too? Can't find anything for it in the GitHub client

[bluca]

Ok, I can squash them when merging

[Asmod4n]

Btw, std::chrono isn't useable because it would require a too new c++ Version?

[bluca]

Now a few tests fail:

dyld: lazy symbol binding failed: Symbol not found: __Z17alt_clock_gettimeiP8timespec
  Referenced from: /Users/travis/build/zeromq/libzmq/zeromq-4.2.3/_build/sub/src/.libs/libzmq.5.dylib
  Expected in: flat namespace

[bluca]

That looks like it's C++11 so yes, too new, sorry we got users on Solaris, AIX and the like

[Asmod4n]

Saw that, sorry for that. Thought its only used in clock.cpp and not exported. Btw, why is it exported?

[bluca]

There's a reference in src/condition_variable.hpp as well

[bluca]

Originally it was without the alt_ prefix and it was just a substitute for the missing system call. Then Apple added it, and it got mangled a bit. I'll clean it up one day...

[Asmod4n]

Oh, SublimeText hasn't found it there.
Updated it so the clock_id gets passed down to the mach function.

[bluca]

why calendar clock? I think we want monotonic

[Asmod4n]

clock_gettime uses the wall clock too there.

[Asmod4n]

Could you cancel all builds then please from this pr? Ill change it all to monotonic clocks then.

[bluca]

thanks - this is a wrapper around pthread conditions, so monotonic clock is the right thing

[bluca]

killed the builds