[Swan] useless gcc "warning: comparison is always false due to limited range of data type [-Wtype-limits]"

[D. Hugh Redelmeier]

| From: Paul Wouters <paul at nohats.ca>

| Can we add it via a #pragma within the file itself? If we add a flag, I
| fear we might lose it some time in the future.

Apparently so:
<http://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html>

Awkward but workable.  I'll give it a try.