generate pkgconfig files, install headers
... so downstream packages can require libraries and headers for gioenvironmentproxy
gioopenssl
giognutls
giognomeproxy
gioproxy
for example, frida-core requires gioopenssl
which is currently not found by pkg-config
frida has the fork https://github.com/frida/glib-networking where they install pkgconfig file and header for gioopenssl, see https://github.com/frida/glib-networking/commit/26361cfb9c19c1adaa8983542236482e2aa641d1#diff-01f733251d19e8c958f8ba33381a3b856648b912a1fc2427f611f1c89493803d
meson docs: https://mesonbuild.com/Pkg-config-files.html and https://mesonbuild.com/Pkgconfig-module.html