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 <>
2 jobs for fixes in 1 minute and 52 seconds (queued for 1 second)