Refactor Config.pm to set @EXPORT_OK from keys %Export_Cache

Refactor Config.pm to set @EXPORT_OK from keys %Export_Cache

The refactor generates %Export_Cache at perl build time, and hence avoids
Config.pm running a map on load. The change also results in @Config::EXPORT
containing shared hash key scalars, which saves 124 bytes on this platform,
modest, but everything helps.

Also change the build script to programmatically generate the function stubs
in Config.pm, instead of having the list duplicated by hand.