Deterministic OSX Dmg.

What is this?
This is a set of helper files to allow the creation of a Deterministic OSX
Bitcoin-Qt installer from Linux. At the time of this writing it is able to
produce a working, deterministic dmg (Tested on 10.6 and 10.8), but it is
a proof of concept, and _very_ ugly.

Why?
Two things were needed:
1. A way to build OSX Bitcoin binaries from Linux, for easier testing and
   automation.
2. A deterministic dmg for stable releases, so that users don't have to trust
   that their binaries aren't spoofed.

These two things are combined here.

How?
Using a combination of Clang and Apple's open-source build tools.

Clang understands darwin-specific options such as -arch, -mmacosx-version-min,
and Framework handling. Unfortunately, Clang relies on GNU ld (for now), so
these object files aren't very useful with Clang alone. To fix this, Apple's
binutils (cctools in Apple-speak) are built on the host machine. Clang is then
directed towards these tools.

There are 2 phases to the build-process. First, build-side tools are built with
the system's existing toolchain. They are installed to their own prefix. After
that, the target libraries can be built with them, as well as Bitcoin itself.
The target libs are installed to their own prefix. Bitcoin's build is then
pointed at the target prefix to find its dependencies.

All libs are built static except for Qt, since it is much better supported as a
framework.

Bitcoin-Qt runs with no obvious regressions and bitcoind and bitcoin-qt tests
pass.

Hurdles and Solutions:

- Apple's SDK is required for headers and frameworks. It is free to download,
  but cannot be redistributed. It can be acquired by registering a free
  account, grabbing the full SDK, and extracting the necessary package.

- Most libs can't handle being built in Linux, so they have been extensively
  patched. Most of these patches come from Macports. Thanks Macports!

- Clang requires the "-target" flag for cross builds. This has been patched in
  in many places.

- Clang can't find a working linker by default. The undocumented "-B" flag is
used, which mimic's gcc's behavior.

- Apple's ar/libtool aren't deterministic. The undocumented environment
  variable ZERO_AR_DATE is set, which zeroes out timestamps and fixes that.

- DMGs are rough to build. Really rough. genisoimage (mkisofs) is used to
  create a very simple one. It is non-deterministic, but faketime works to fix
  that.

This can all be automated using Gitian. Descriptors have been provided.

Howto:

1. Acquire the SDK. See the note in download.sh for more info.
2. Run ./download.sh to grab all necessary sources.
3. Run ./build.sh

   All necessary dependencies are now installed in prefix.


Respected environment variables:
- $PREFIX: Prefix where target files will be installed.
- $NATIVEPREFIX: Prefix where host libs and tools will be installed.
- $SOURCES_PATH: Where the source tarballs should be found.
  default paths.


TODO:

- Evaluate whether it's necessary to be able to build outside of Gitian. If so,
  these Makefiles need an overhaul.

- Prettify the dmg installer. DMGs are typically customized by mounting,
  decorating, and unmounting. This is not possible in Linux. Instead, it is
  done in OSX and the customization file (.DS_Store) is saved and injected
  into the DMG. This is currently working but has a few rough edges.

- DMGs are uncompressed. There are a few options for fixing this, they need
  further investigation.

- Test in 10.5 and 10.9.

- Move openssl to a dylib. It is linked by bitcoin-qt as well as the qt
  framework, so it exists twice in the final dmg.

Bounty/Donation address: 17uFa79oMQD6X8Tw4WL7ceFJY2zJJFGfrr