Handle configure.pkg_config_path as a list

[Ryan Schmidt]

MacPorts base sets PKG_CONFIG_PATH to ${configure.pkg_config_path}.

But should we allow configure.pkg_config_path to be a Tcl list, and join the elements with a ":" before assigning it to PKG_CONFIG_PATH?

I don't see any ports currently needing to set it to more than a single path, but if any did need that, this would make it much easier for them.