build: Fix clock_gettime check with XCode 8

With XCode 8, clock_gettime will be incorrectly detected as being
available regardless of what OS X version we're targetting because the
symbol is available in the .tbd library as a weak symbol.
See: https://github.com/Homebrew/homebrew-core/issues/3727#issue-170086273

It's only starting from macOS 10.12 that clock_gettime is actually
available, so we can unconditionally disable it when targetting older
versions. We cannot simply do AC_CHECK_FUNCS with -Wl,-no_weak_imports
because the autoconf check does its own prototype declaration that
doesn't trigger that compiler flag.

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

configure.ac

@@ -671,18 +671,50 @@ AC_CHECK_FUNCS([posix_memalign])
 AC_CHECK_FUNCS([getpagesize])
 
 dnl Check for POSIX timers
-AC_CHECK_FUNCS(clock_gettime, [], [
+CLOCK_GETTIME_FOUND="no"
+AC_CHECK_FUNC(clock_gettime, [CLOCK_GETTIME_FOUND="yes"], [
   AC_CHECK_LIB(rt, clock_gettime, [
-    AC_DEFINE(HAVE_CLOCK_GETTIME, 1)
+    CLOCK_GETTIME_FOUND="yes"
     LIBS="$LIBS -lrt"
   ], [
     AC_CHECK_LIB(pthread, clock_gettime, [
-      AC_DEFINE(HAVE_CLOCK_GETTIME, 1)
+      CLOCK_GETTIME_FOUND="yes"
       LIBS="$LIBS -lpthread"
     ])
   ])
 ])
 
+# With XCode 8, clock_gettime will be incorrectly detected as being available
+# regardless of what version of OS X you target because the symbol is available
+# in the .tbd file as a weak symbol.
+# See: https://bugzilla.gnome.org/show_bug.cgi?id=772451 
+#
+# We cannot simply do AC_CHECK_FUNCS with -Wl,-no_weak_imports because the
+# autoconf check does its own prototype declaration that doesn't trigger that
+# compiler flag.
+#
+# It's only starting from macOS 10.12, that clock_gettime is actually available,
+# so we can unconditionally disable it when targetting older versions.
+case "$host_os" in
+  darwin*)
+    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+#include <AvailabilityMacros.h>
+#if MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12
+#error "Not compiling for OS X 10.12 or later"
+#endif
+    ]])], [], [
+    if test "$CLOCK_GETTIME_FOUND" = "yes"; then
+      AC_MSG_NOTICE([Disabling incorrectly detected clock_gettime on OS X])
+    fi
+    CLOCK_GETTIME_FOUND="no"
+    ])
+    ;;
+esac
+
+if test "$CLOCK_GETTIME_FOUND" = "yes"; then
+  AC_DEFINE(HAVE_CLOCK_GETTIME, 1, [Have clock_gettime])
+fi
+
 AC_CACHE_CHECK(for posix timers, gst_cv_posix_timers,
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 #include <time.h>

m4/check-checks.m4

@@ -95,7 +95,7 @@ AC_CHECK_LIB([rt], [timer_create, timer_settime, timer_delete])
 AM_CONDITIONAL(HAVE_TIMER_CREATE_SETTIME_DELETE, test "x$ac_cv_lib_rt_timer_create__timer_settime__timer_delete" = "xyes")
 
 dnl Allow for checking HAVE_CLOCK_GETTIME in automake files
-AM_CONDITIONAL(HAVE_CLOCK_GETTIME, test "x$ac_cv_func_clock_gettime" = "xyes")
+AM_CONDITIONAL(HAVE_CLOCK_GETTIME, test "x$HAVE_CLOCK_GETTIME" = "xyes")
 
 dnl Create _stdint.h in the top-level directory
 AX_CREATE_STDINT_H

meson.build

@@ -2,7 +2,7 @@ project('gstreamer', 'c', 'cpp',
   version : '1.9.90',
   meson_version : '>= 0.33.0',
   default_options : [ 'warning_level=1',
-                      'c_std=gnu99',
+                      'c_std=c99',
                       'buildtype=debugoptimized' ])
 
 gst_version = meson.project_version()
@@ -185,8 +185,30 @@ endif
 if cc.has_function('getpagesize', prefix : '#include<unistd.h>')
   cdata.set('HAVE_GETPAGESIZE', 1)
 endif
+
+# With OS X 10.11.6 and XCode 8, clock_gettime will be incorrectly detected as
+# being available because the symbol is available in the .tbd file as a weak
+# symbol. See: https://github.com/Homebrew/homebrew-core/issues/3727#issue-170086273
+#
+# We cannot simply do cc.has_function with -Wl,-no_weak_imports because the
+# check does its own prototype decl that doesn't trigger that compiler flag.
+#
+# It's only starting from macOS 10.12, that clock_gettime is actually available,
+# so we can unconditionally disable it for older versions.
+for_osx_10_12_src = '''#include <AvailabilityMacros.h>
+#if MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12
+#error "Not compiling for OS X 10.12 or later"
+#endif
+'''
+
 if cc.has_function('clock_gettime', prefix : '#include <time.h>')
-  cdata.set('HAVE_CLOCK_GETTIME', 1)
+  if host_machine.system() == 'darwin'
+    if cc.compiles(for_osx_10_12_src, name : 'minimum OS X version required >= 10.12')
+      cdata.set('HAVE_CLOCK_GETTIME', 1)
+    endif
+  else
+    cdata.set('HAVE_CLOCK_GETTIME', 1)
+  endif
 endif
 
 # We only want to use the __declspec(dllexport/import) dance in GST_EXPORT when