meson: Use pkgconfig.generate

Part-of: <gstreamer/orc!49>
5 jobs for pkgconfig in 3 minutes and 35 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #3854922
debian sid

00:00:35

passed #3854932
gst-macos-10.15
meson macos

00:00:33

manual #3854930
gstreamer-windows 1809 windows docker allowed to fail manual
msys2
passed #3854924
gstreamer-windows 1809 windows docker
vs2017 amd64

00:02:46

passed #3854927
gstreamer-windows 1809 windows docker
vs2017 x86

00:01:07