Generate build.mk for bindists

The config.mk.in script was relying on some variables which were
supposed to be set by build.mk but therefore never were when used to
install a bindist.

Specifically

* BUILD_PROF_LIBS to determine whether we had profiled libraries or not
* DYNAMIC_GHC_PROGRAMS to determine whether we had shared libraries or
  not

Not only were these never set but also not really accurate because you
could have shared libaries but still statically linked ghc executable.

In addition variables like GhcLibWays were just never used, so those
have been deleted from the script.

Now instead we generate a build.mk file which just directly specifies
which RtsWays we have supplied in the bindist and whether we have
DYNAMIC_GHC_PROGRAMS.