Ocb_stubblr.Pkg_configQuery pkg-config.
pkg-config is invoked with the environment extended with the equivalent of
PKG_CONFIG_PATH=$(opam config var prefix)/{lib,share}/pkgconfig. This means that any .pc files in the current Opam switch take precedence over the system-wide ones.
run ~flags package queries pkg-config about package, using flags.
`Nonexistent means that the package was not found. Otherwise, `Res output is whatever pkg-config prints on the standard output.
Note Currently, all errors in pkg-config invocation results in `Nonexistent.