Check for -Wno-unused-but-set-variable gcc support

Gcc versions 4.3.2 and earlier do not support the compiler flag
-Wno-unused-but-set-variable. This can lead to build failures
on older Linux platforms such as Debian Lenny. Since this is
an optional build argument this changes add a new autoconf check
for the option. If it is supported by the installed version of
gcc then it is used otherwise it is omited.

See commit's 12c1acde76683108441827ae9affba1872f3afe5 and
79713039a2b6e0ed223d141b4a8a8455f282d2f2 for the reason the
-Wno-unused-but-set-variable options was originally added.