[PATCH] Suppress Clang 3.9 warnings

Hello.

I've just tried to build PostgreSQL with Clang 3.9.1 (default version
currently available in Arch Linux) and noticed that it outputs lots of
warning messages. Most of them are result of a bug in Clang itself:

```
postinit.c:846:3: note: include the header <string.h> or explicitly
provide a declaration for 'strlcpy'
```

I've reported it to Clang developers almost a year ago but apparently
no one cares. You can find all the details in a corresponding thread
[1]. Frankly I'm not sure what to do about it. 

The rest of warnings looks more like something we could easily deal with:

```
xloginsert.c:742:18: warning: implicit conversion from 'int' to 'char'
changes value from 253 to -3 [-Wconstant-conversion]
```

Patch that fixes these warnings is attached to this email.

[1] http://lists.llvm.org/pipermail/cfe-dev/2016-March/048126.html

-- 
Best regards,
Aleksander Alekseev