Bite the bullet and make GHCi support non-optional in the RTS. GHC
4.11 should be able to build GHCi without any additional tweaks now.
- the Linker is split into two parts: LinkerBasic.c, containing the
routines required by the rest of the RTS, and Linker.c, containing
the linker proper, which is not referred to from the rest of the RTS.
Only Linker.c requires -ldl, so programs which don't make use of the
linker (everything except GHC, in other words) won't need -ldl.