Add osx test for the XCode 8 fixes to has_function

test cases/osx/3 has function xcode8/meson.build

@@ -0,0 +1,26 @@
+project('has function xcode8', 'c')
+
+cc = meson.get_compiler('c')
+
+# XCode 8 location for the macOS 10.12 SDK
+sdk_args = ['-isysroot', '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk']
+args_10_11 = ['-mmacosx-version-min=10.11'] + sdk_args
+args_10_12 = ['-mmacosx-version-min=10.12'] + sdk_args
+
+# Test requires XCode 8 which has the MacOSX 10.12 SDK
+if cc.version().version_compare('>=8.0')
+  if cc.has_function('clock_gettime', args : args_10_11, prefix : '#include <time.h>')
+    error('Should not have found clock_gettime via <time.h> when targetting Mac OS X 10.11')
+  endif
+  if not cc.has_function('clock_gettime', args : args_10_12, prefix : '#include <time.h>')
+    error('Did NOT find clock_gettime via <time.h> when targetting Mac OS X 10.12')
+  endif
+  if not cc.has_function('clock_gettime', args : args_10_11)
+    error('Did NOT find clock_gettime w/o a prototype when targetting Mac OS X 10.11')
+  endif
+  if not cc.has_function('clock_gettime', args : args_10_12)
+    error('Did NOT find clock_gettime w/o a prototype when targetting Mac OS X 10.12')
+  endif
+else
+  message('Test needs XCode 8, skipping...')
+endif