pg_config is deprecated in some distros

[sadko4u]

For some Linux Distributions, pg_config tool is deprecated.
Maintainers recommend to use standard pkg-config tool instead.
Would be nice if you could add the corresponding workaround.

https://lists.opensuse.org/opensuse-factory/2019-03/msg00299.html

[dvarrazzo]

I don't use suse. If you do documentation patch is welcome.

[sadko4u]

I don't use Debian-based distros nor FreeBSD. But when necessary, I install a virtual machine:

lsp-plugins/lsp-plugins#154

Moreover, I'm not a python developer do deeply dig into code and build process of the library.

[dvarrazzo]

Duplicate of #1001