pkg-config: Only append -lm if necessary.
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 <98bd4b740b8603b8d33970ec1a6111b4e24a56a2@gmail.com>

ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis