Stage1Only conditional program/package building
This patch includes changes to allow programs and packages to be disabled by stage when building GHC.
- Add guards for "disable-$(dir)-$(stage)" around the rule body
- Add guards for "disabled-$(dir)-$(stage)" around the rule body
- Add a set of package disabling definitions when $(Stage1Only) is set to "YES"
- Add the all_rts target as a dependency for all, causing the rts to build when packages that depend on it are disabled