Fix pkgconfig creation with cmake.

The cmake build script was not setting a VORBIS_LIBS variable
that is used to update pkg-config files. This results in linking
errors in downstream projects due to missing dependencies
(in this case libm).

This patch just updates the cmake script to behave the same
as configure does currently.

Signed-off-by: evpobr <evpobr@gmail.com>
2 jobs for cmake in 1 minute and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10935
docker
autoconf

00:01:46

passed #10936
docker
cmake

00:00:29