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>
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