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This required various build system changes to get the build to go
through.

In the inplace shell wrappers, we set LD_LIBRARY_PATH to allow programs
to find their libraries. In the future, we might change the inplace tree
to be the same shape as an installed tree instead. However, this would
mean changing the way we do installation, as currently we use cabal's
installation methods to install the libraries, but that only works if
the libraries are under libraries/foo/dist-install/build/..., rather
than in inplace/lib/...

Now that it's included in settings, the bindist configure script needs
to know it.

Related to #4862

Maybe we could revisit whether we want to allow building bindists of
cross-compilers later, but for now I'm fixing validate by setting all
the cross-compilation settings to NO.

We want to use the inplace ar, rather than whichever ar the
machine that we build on happens to have.

Now target32bit works for all targets without any manual intervention,
as it should do.  #5735 was a portability regression.

Some platforms use linkers that don't support the --hash-size=31 and
--reduce-memory-overheads flags.

In a bindist, we generate files like the hsc2hs wrapper.
This means we need to have the right values for the variables like
CONF_GCC_LINKER_OPTS_STAGE1 which in turn means we need to know what
platform we're on.

rather then the bootstrapping compiler. This fixes problems where the
bootstrapping compiler dynamically links against libraries not on the
target machine.

This means we don't rely on baking a path to gcc into the executable

We already had a check in configure of the actual build. This patch
adds it to the configure of the bindists too.

Also tidied up the way configure.ac gets into bindists

This variable is tested when deciding whether or not to add debug to
the list of RTS ways, so it needs to be correctly defined.

This means that PACKAGE_TARNAME gets set, so the default docdir is
correct.

Also fixes it so that it really does pass (we weren't substituting the
GMP variables in the RTS package config).

We shouldn't need it, as we don't call cygwin's gcc, and it was causing
problems with the nightly builders passing it to GHC.

We now have various flags that need to be passed to gcc, but only if
the local gcc version supports them.  So instead of wiring this
knowledge into ghc when it is built, we now put these "extra gcc
flags" into a text file in $libdir, extra-gcc-flags, which is created
by configure.  It is also created by the configure script of a binary
distribution, so a binary dist is independent of the gcc version used
to build it.

Hopefully I got the binary-dist stuff right, but binary dists aren't
currently working so we'll need to come back and check this.

Bindists should now work again, when doing "make install" at least.
"make in-place" is probably still broken.

Basic -fvia-C code generation is there, not much testing.

Support i386-apple-darwin for binary dists.

MERGE TO STABLE

configure-bin.ac: I didn't know this file existed.
                  Add powerpc-linux and powerpc64-linux targets.

Add x86_64 platforms