Building statsite with older gcc (4.1.2) fails with dereferencing type-punned pointer will break strict-aliasing rules

There are no warnings whatsoever with newer gcc (4.7.3). 

Feel free to reject this pull request :) I understand that compatibility with ancient compilers is not a priority. 

```
$ gcc --version
gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-44)
...
cc1: warnings being treated as errors
src/cm_quantile.c: In function 'cm_insert':
src/cm_quantile.c:254: warning: dereferencing type-punned pointer will break strict-aliasing rules
src/cm_quantile.c:274: warning: dereferencing type-punned pointer will break strict-aliasing rules
src/cm_quantile.c:275: warning: dereferencing type-punned pointer will break strict-aliasing rules
src/cm_quantile.c:293: warning: dereferencing type-punned pointer will break strict-aliasing rules
src/cm_quantile.c:294: warning: dereferencing type-punned pointer will break strict-aliasing rules
```