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This may not be ideal, but it should get GHC building on all platforms
again.

Apparently on Solaris it is an error to omit _ISOC99_SOURCE when using
_POSIX_C_SOURCE==200112L.

With glibc, things like _POSIX_C_SOURCE and _ISOC99_SOURCE are
additive, but on FreeBSD they are mutually exclusive.  However, it
turns out we only need to define _POSIX_C_SOURCE and _XOPEN_SOURCE to
get all the C99 stuff we need too, so there's no need for any #ifdefs.

Submitted by: Gabor PALI <pgj@FreeBSD.org>

The first phase of this tidyup is focussed on the header files, and in
particular making sure we are exposinng publicly exactly what we need
to, and no more.

 - Rts.h now includes everything that the RTS exposes publicly,
   rather than a random subset of it.

 - Most of the public header files have moved into subdirectories, and
   many of them have been renamed.  But clients should not need to
   include any of the other headers directly, just #include the main
   public headers: Rts.h, HsFFI.h, RtsAPI.h.

 - All the headers needed for via-C compilation have moved into the
   stg subdirectory, which is self-contained.  Most of the headers for
   the rest of the RTS APIs have moved into the rts subdirectory.

 - I left MachDeps.h where it is, because it is so widely used in
   Haskell code.
 
 - I left a deprecated stub for RtsFlags.h in place.  The flag
   structures are now exposed by Rts.h.

 - Various internal APIs are no longer exposed by public header files.

 - Various bits of dead code and declarations have been removed

 - More gcc warnings are turned on, and the RTS code is more
   warning-clean.

 - More source files #include "PosixSource.h", and hence only use
   standard POSIX (1003.1c-1995) interfaces.

There is a lot more tidying up still to do, this is just the first
pass.  I also intend to standardise the names for external RTS APIs
(e.g use the rts_ prefix consistently), and declare the internal APIs
as hidden for shared libraries.

Most of the other users of the fptools build system have migrated to
Cabal, and with the move to darcs we can now flatten the source tree
without losing history, so here goes.

The main change is that the ghc/ subdir is gone, and most of what it
contained is now at the top level.  The build system now makes no
pretense at being multi-project, it is just the GHC build system.

No doubt this will break many things, and there will be a period of
instability while we fix the dependencies.  A straightforward build
should work, but I haven't yet fixed binary/source distributions.
Changes to the Building Guide will follow, too.

Big re-hash of the threaded/SMP runtime

This is a significant reworking of the threaded and SMP parts of
the runtime.  There are two overall goals here:

  - To push down the scheduler lock, reducing contention and allowing
    more parts of the system to run without locks.  In particular,
    the scheduler does not require a lock any more in the common case.

  - To improve affinity, so that running Haskell threads stick to the
    same OS threads as much as possible.

At this point we have the basic structure working, but there are some
pieces missing.  I believe it's reasonably stable - the important
parts of the testsuite pass in all the (normal,threaded,SMP) ways.

In more detail:

  - Each capability now has a run queue, instead of one global run
    queue.  The Capability and Task APIs have been completely
    rewritten; see Capability.h and Task.h for the details.

  - Each capability has its own pool of worker Tasks.  Hence, Haskell
    threads on a Capability's run queue will run on the same worker
    Task(s).  As long as the OS is doing something reasonable, this
    should mean they usually stick to the same CPU.  Another way to
    look at this is that we're assuming each Capability is associated
    with a fixed CPU.

  - What used to be StgMainThread is now part of the Task structure.
    Every OS thread in the runtime has an associated Task, and it
    can ask for its current Task at any time with myTask().

  - removed RTS_SUPPORTS_THREADS symbol, use THREADED_RTS instead
    (it is now defined for SMP too).

  - The RtsAPI has had to change; we must explicitly pass a Capability
    around now.  The previous interface assumed some global state.
    SchedAPI has also changed a lot.

  - The OSThreads API now supports thread-local storage, used to
    implement myTask(), although it could be done more efficiently
    using gcc's __thread extension when available.

  - I've moved some POSIX-specific stuff into the posix subdirectory,
    moving in the direction of separating out platform-specific
    implementations.

  - lots of lock-debugging and assertions in the runtime.  In particular,
    when DEBUG is on, we catch multiple ACQUIRE_LOCK()s, and there is
    also an ASSERT_LOCK_HELD() call.

What's missing so far:

  - I have almost certainly broken the Win32 build, will fix soon.

  - any kind of thread migration or load balancing.  This is high up
    the agenda, though.

  - various performance tweaks to do

  - throwTo and forkProcess still do not work in SMP mode

forgot to add this file