check: Fix macro check for OS X

TARGET_OS_MAC is defined on all Apple platforms. You need to check for
!TARGET_OS_IPHONE to detect OS X (now called macOS).

libs/gst/check/libcheck/libcompat/clock_gettime.c

@@ -25,7 +25,8 @@
 #include <mach/mach.h>
 #include <mach/mach_time.h>
 #include <TargetConditionals.h>
-# if TARGET_OS_MAC
+/* CoreServices.h is only available on macOS */
+# if TARGET_OS_MAC && !TARGET_OS_IPHONE
 #  include <CoreServices/CoreServices.h>
 # endif
 #include <unistd.h>