DYNAMIC_GHC_PROGRAMS and GhcDynamic both tried to control whether
the ghc binary was built as a dynamic executable, with confusing
results. In particular, setting GhcDynamic=NO has no effect on
systems where DYNAMIC_GHC_PROGRAMS defaults to YES.
DYNAMIC_GHC_PROGRAMS is more fully-featured (it ensures that the
correct flavor of the libraries is built, for example) so let's
keep it and remove GhcDynamic to reduce confusion.
This effectively reverts commit 3c6190b0.