[Bug 772451] Broken clock_gettime and mkostemp detection with XCode 8

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=772451

--- Comment #17 from Nirbheek Chauhan <nirbheek.chauhan at gmail.com> ---
I forgot to add another thing (I wish you could edit bugzilla comments): 

Apple wants you to use the new symbols as weak imports so that you can compile
a single binary that can run on multiple platforms. You are supposed to check
if the symbol is available at runtime and use fallback code if it does not.

I think this is a really cool feature, but they added it in a terrible way.
They made it the default for all symbols. This means that people will compile
their app with a new SDK and it will barf when they run it on an older OS X.
This is most prominently visible on OS X 10.11.6 where if you build with the
default XCode 8 SDK, your program will use symbols not available on your OS at
all and fail at runtime.

To mitigate this, they added a new option that forces the compiler to not
assume that your symbol usage is proper, and error out if you have any
references to symbols not available in your targetted OS X version:
`-Wl,-no_weak_imports`.

If you use that to compile your basic testcase, it will fail as follows:

$ clang -Wl,-no_weak_imports -mmacosx-version-min=10.8 -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk
test2.c
ld: weak import of symbol '_clock_gettime' not supported because of option:
-no_weak_imports for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
$


And it will work fine if you target a newer OS X:

$ clang -Wl,-no_weak_imports -mmacosx-version-min=10.12 -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk
test2.c
$

BUT, this option relies on availability information in the prototype (time.h
for clock_gettime). So if you don't use the prototype in the header provided by
Apple, the symbol will always be "available" to the linker. This is why the
Autoconf check is broken no matter what we do and we have to work around it
manually -- it always uses its own broken prototype.

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