configure: try to disable weak linking on OSX

configure: try to disable weak linking on OSX

macOS 10.12 introduced support for clock_gettime() but when compiling
with Xcode 8 (or later) and targeting 10.11 (or older) the clock_gettime
symbol is now found at configure/compile time but referenced weakly.
Running the created binary on 10.11 results in a "dyld: lazy symbol
binding failed: Symbol not found: _clock_gettime" error.

This drama has played out across other projects:
- https://github.com/Homebrew/homebrew-core/issues/2674
- https://github.com/curl/curl/issues/1069
- https://svn.filezilla-project.org/filezilla?view=revision&revision=7824OO
- https://github.com/mesonbuild/meson/pull/949
- https://github.com/Homebrew/homebrew-core/issues?utf8=%E2%9C%93&q=label%3Aclock_gettime%20

Options for working around this issue include:
1. Turn references to functions that might be weak into compilation
   errors (-Werror=partial-availability).
2. Disable visibility of functions that might be weak at compile link
   time (-Wl,-no_weak_imports).
3. Change configure tests to directly check the targeted OSX version and
   fail the test if it is too old to have the required function.
4. Make an empty function declaration marked with
   __attribute__((weak_import)) that is used if the symbol would
   otherwise be unavailable. It is then possible to check if the symbol
   is NULL at runtime to determine availability.

This commit does 2. when targeting the Darwin platform after checking if
the compiler/linker work with that option. It also changes the clockid_t
configure test to no longer use clock_gettime() because targeting OSX
10.11 when using the 10.12 SDK results in having the clockid_t typedef
without having the clock_gettime function. The combination of these
fixes should solve https://github.com/axboe/fio/issues/305 .

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>