threads,configure.ac,meson.build: define and use HAVE_TIMESPEC_GET

Tested with Travis and Appveyor.

v2: add HAVE_TIMESPEC_GET for non-Windows Scons builds
v3: use check_functions in Scons (Eric)

Cc: Rob Herring <robh@kernel.org>
Cc: Alexander von Gluck IV <kallisti5@unixzen.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103674
Fixes: f1a3648784 ("threads: update for late C11 changes")
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Jon Turney <jon.turney@dronecode.org.uk> (v2)

meson.build

@@ -578,7 +578,7 @@ foreach h : ['xlocale.h', 'sys/sysctl.h', 'linux/futex.h']
   endif
 endforeach
 
-foreach f : ['strtof', 'mkostemp', 'posix_memalign']
+foreach f : ['strtof', 'mkostemp', 'posix_memalign', 'timespec_get']
   if cc.has_function(f)
     pre_args += '-DHAVE_@0@'.format(f.to_upper())
   endif