build: Use cmake to look up abseil dependency

This should be much more robust than looking up the library directly.

Fixes: pulseaudio/webrtc-audio-processing#4
4 jobs for vendor-abseil in 2 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Container
passed #6111073
aarch64
build-container-aarch64

00:01:21

passed #6111072
build-container-x86_64

00:01:20

 
  Build
passed #6111075
aarch64
build-aarch64

00:00:49

passed #6111074
build-x86_64

00:00:47