i18n: Don't compile with -ffast-math

This flag results in calls to (at least) isfinite() and isnan() becoming
skipped, and a constant false returned. This caused volume-test to fail
on Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=916504

Since PulseAudio deals with negative infinities with volume dB values,
this is not a problem only in volume-test. We shouldn't use -ffast-math
at all.
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  Build
passed build-autotools #322441

00:08:16

passed build-meson #322442

00:00:27