[make] let user know how to stop clang rebuilds
[make] let user know how to stop clang rebuilds

Summary:
Building clang takes long and sometimes only some minor step in the setup
script has changed, triggering a spurious rebuild. Print a big bold message
letting the user know of their options, but only if clang has been installed
previously.

```
(warn_clang)jul@devvm2701:~/infer$ make -j 19
[04:02:44][2288770] Facebook setup...
*** Now building clang, this will take a while...
*** If you believe that facebook-clang-plugins/clang/install is up-to-date you can(TERM_RESET)
*** interrupt the compilation (Control-C) and run this to prevent clang from being rebuilt:

      /home/jul/infer/facebook-clang-plugins/clang/setup.sh --only-record-install

(TIP: you can also force a clang rebuild by removing /home/jul/infer/facebook-clang-plugins/clang/installed.version)

[...]
```

Reviewed By: katiejots

Differential Revision: D13956431

fbshipit-source-id: da163643d

jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer