all around: disable print_system_libs in calls to pkg-config

By default, pkg-config-rs sets the PKG_CONFIG_ALLOW_SYSTEM_LIBS environment variable (https://github.com/alexcrichton/pkg-config-rs/pull/35), which leads to eg /usr/lib being present in the middle of the final command, in turn leading to issues when working in an uninstalled environment, where libraries that are also present system-wide do not get linked against.