Fix legacy builds crashing in 10.9-10.11 by disabling clock_gettime

Recent Vim builds introduced the use of clock_gettime(), which on macOS
was only introduced in 10.12. In Vim, we added a configure check to
detect its existence (vim/vim#12242) but it
only works to detect the build-time environment. For MacVim builds we
build on new machines but deploy on old machines by using
MACOSX_DEPLOYMENT_TARGET=10.9 when building. This means that the
configure script will detect clock_gettime() exists and includes it, but
at runtime the app will crash when running MacVim (legacy builds) in
10.9-10.11.

To fix this, add explicit checks to make sure we undef
HAVE_CLOCK_GETTIME if deployment target is below 10.12. Unfortunately
there isn't really a way to detect such things automatically in a
configure script as it requires knowledge of historical releases.

src/os_mac.h

@@ -264,6 +264,14 @@ typedef int clockid_t;
 #  define CLOCK_MONOTONIC 1
 # endif
 
+# if !defined(MAC_OS_X_VERSION_10_12) \
+	|| (MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12)
+// This happens when building on a newer machine for a min deployment lower
+// than 10.12. The build environment supports clock_gettime(), but the target
+// runtime doesn't.
+#  undef HAVE_CLOCK_GETTIME
+# endif
+
 struct itimerspec
 {
     struct timespec it_interval;  // timer period