configure.ac : Include a couple more -W flags.
tests/floating_point_test.tpl : Reduce tolerance of a couple of tests.

A couple of tests were failing when compiled with gcc-4.8 (and
gcc-snapshot/4.9.0 from Debian). The tests changed were for 32 bit
ALAC (test result changed from -186 to -181 dB signal-to-noise) and
for doubles (test results changed from -300 to -205 dB signal-to-noise).
configure.ac : Fix for current versions of autotools.

Patch from Cristian Rodriguez.