pkg-config: Only append -lm if necessary.

Some systems don't require a separate -lm to link with libc
math functions. The configure script checks for this, so
use the corresponding variable to set the dependent libraries
in vorbis.pc dynamically.

Signed-off-by: Marvin Scholz <epirat07@gmail.com>
2 jobs for fixes in 1 minute and 52 seconds (queued for 1 second)
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passed #10308
docker
autoconf

00:01:52

passed #10309
docker
cmake

00:00:37