Unused variables warning with -Wextra flag

[bdsatish]

Hi,

Some warning messages are being "leaked" from GDC's source code 
into end-user's code. I think this shouldn't happen and hence, is 
a bug.

```
// example.d
import std.string;

void main()
{
   format("Hello World");
}

```
Here's the output:

```
[shell]$ gdc -Wextra example.d
$INSTALLDIR/gcc/include/d/4.8.0/std/utf.d: In function 'decode':
$INSTALLDIR/gcc/include/d/4.8.0/std/utf.d:856: warning: 
uninitialized variable 'result'
$INSTALLDIR/gcc/include/d/4.8.0/std/utf.d: In function 'decode':
$INSTALLDIR/gcc/include/d/4.8.0/std/utf.d:856: warning: 
uninitialized variable 'result'
$INSTALLDIR/gcc/include/d/4.8.0/std/utf.d: In function 'decode':
$INSTALLDIR/gcc/include/d/4.8.0/std/utf.d:856: warning: 
uninitialized variable 'result'
example.d: In function 
'std.format.formattedWrite!(Appender!(string), char, 
).formattedWrite':
$INSTALLDIR/gcc/include/d/4.8.0/std/format.d:504: warning: 
'iftmp.16' may be used uninitialized in this function
$INSTALLDIR/gcc/include/d/4.8.0/std/format.d:504: warning: 
'iftmp.17' may be used uninitialized in this function
```

So, all these warnings seem to be from GDC. Is there a way to 
supress these warnings? (for GDC's libs, not my own code).


--Satish