Avoid clock_gettime on pre-10.12 macOS versions

On older macOS like 10.10 we saw the following compiler error:

```
/go/src/github.com/cockroachdb/cockroach/c-deps/rocksdb/env/env_posix.cc:845:19:
error: use of undeclared identifier 'CLOCK_THREAD_CPUTIME_ID'
    clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
                  ^
```

According to mac's `man clock_gettime`: "These functions first appeared in Mac
OSX 10.12". So we should not try to compile it on earlier versions.

Test Plan: verified it compiles now on 10.10. Also did some investigation to
ensure it does not cause regression on macOS 10.12+, although I do not
have access to such an environment to really test.

env/env_posix.cc

@@ -33,6 +33,7 @@
 // Get nano time includes
 #if defined(OS_LINUX) || defined(OS_FREEBSD)
 #elif defined(__MACH__)
+#include <Availability.h>
 #include <mach/clock.h>
 #include <mach/mach.h>
 #else
@@ -839,7 +840,7 @@ class PosixEnv : public Env {
 
   uint64_t NowCPUNanos() override {
 #if defined(OS_LINUX) || defined(OS_FREEBSD) || defined(OS_AIX) || \
-    defined(__MACH__)
+    (defined(__MACH__) && defined(__MAC_10_12))
     struct timespec ts;
     clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
     return static_cast<uint64_t>(ts.tv_sec) * 1000000000 + ts.tv_nsec;