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Significantly speeds up parallel builds.

Needed for the parallel builds.

It should not be used in packages ready for being distributed.

Fixes the problem with generating the haddock documentation contents page for
all installed packages. Previously we were (accidentally) telling haddock to
use all versions of each package and haddock would pick the first (lowest
version). Now we correctly do what we were trying to do all along, which is
to pick only the highest version of each package.

It adds a '-j N' (= 'number of jobs') option for the 'install' command, which
can be used to specify the number of concurrent workers. If possible, at most
N packages will be built concurrently.

This version of the patch is less featureful than Mikhail's version but also
rather simpler. The key difference compared to Mikhail's version is that this
version is lacking the output serialisation and the ability to tag each output
message with the task it came from. All output is interleaved. The next step
will be to make parallel builds log to files rather than the console and only
to display a summary on the console.

In addition to not having to change the output functions, the code is a bit
simpler by keep the structure of the code the same as before, rather than
splitting it into a number of concurrent tasks with channels. Instead each
task simply executes the same pattern of install actions and concurrency
limits are enforced using semaphores.

We don't want 'InstallPlan.ready' to return packages that are currently
being processed in other threads.

It can be non-obvious why a package is registered more than once.

We don't want FFI calls to block all Haskell threads, so we need to use the
threaded runtime; -rtsopts is for experimenting with the number of capabilities.

I'm looking at you, ansi-terminal.

This saves a bit of heap space when running the solver with unlimited
backtracking enabled.

This saves a lot of heap space while exploring the tree and backtracking.

In particular, this fixes building haskeline with its windows_cconv.h
header.

Fix cabal-install build.

Adapt to package-conf -> package-db change

GHC and ghc-pkg package db flags changed from '*-package-conf' to
'*-package-db' in 7.5.  This commit follows the change and introduces a
version check whenever those flags are used.

Add support for the Apache license

Add .gitignore file and remove .darcs-boring