Implement offline mode for the 'install' command.

Improve a number of ghc-options checks

Currently there are no checks that need to be specific to one of them.
Both sets of options get used in distributed packages.

And extend it to cover other similar variants. People often put it in to
help them profile *this* package, but it causes problems for profiling
other packages. The -fprof-auto-exported is arguably ok though.

The -threaded flag really makes no difference for libraries, it's a
link-time option only.

Improve the package description check for -Werror and similar flags so
that they are allowed so long as they're not enabled by default. We
check the conditions that they're guarded by to make sure they're always
false (given the default values of the manual flags). Also extend the
error messages to explain how to use these flags with conditionals.

[ci skip]

When in offline mode, 'cabal install' only installs packages from the local
tarball cache. Offline mode can be enabled with the '--offline' flag.

Fixes #2566.

Use PVP-compliant example when warning about missing base upper bound

Refs hackage-server#253.

Force cabal upload to always use digest auth and never basic auth

Document more functions in Paths_pkgname, fixes #715

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Move SrcDirs following Main file generator.

Implement a dummy 'uninstall' command.

Doc fix: 'cabal freeze PACKAGES' is not supported.

Fixes #2565.

Make `cabal init` create Main.hs if it doesn't exist.

As suggested by @sclv on reddit. See
https://github.com/haskell/cabal/issues/648#issuecomment-96895441

Print a more friendly message when http_proxy is down.

This reverts commit c9a77353.

Paths_ module: don't require base >= 4.

Fixes #994.

Add Functor, Foldable and Traversable instances to Condition

Rewrite linker check to avoid grepping "ld"s.

I was the one who introduced that bit, and it seems like well may have caused more trouble than it helped... so, sorry :(

Anyway, clang has since then grown the ability to `-print-prog-name`, so I've done away with all that grepping through -### stuff.
@creswick @23Skidoo, could you see if this works for you?

bootstrap.sh: fixes linker matching to avoid cases where tested linker names appear unexpectedly in compiler output (fixes #2542)

bootstrap.sh: fixes linker matching to avoid cases where tested linker names appear unexpectedly in compiler output (fixes #2542)

This fixes cases where strings like "ld" appear in compiler output on lines
that have nothing to do with the linker command invocation.