gitian: don't export any symbols from executable

This avoids conflicts between the libraries statically linked into bitcoin and any
libraries we may link dynamically (such as Qt and OpenSSL, see issue #4094).
It also avoids start-up overhead to not export any unnecessary symbols.
To do this, build a linker script that marks all symbols as local.

Conflicts:

	contrib/gitian-descriptors/gitian-linux.yml
gitian: build against Qt 4.6

Should make it possible to run the resulting GUI executable on
Linux distributions that use Qt 4.6, such as Debian Wheezy and Tails.

Builds a mini-SDK for building against Qt 4.6. This includes the headers
as well as host utilities such as `lrelease`, `qrc` and `moc`.

This speeds up the gitian build a bit - libqt4-dev pulled in a lot of packages,
and is no longer needed as this provides a replacement of our own.

Note: This does not replace the Qt build with at static library. After this
commit we still build dynamically against the system Qt library. The only
difference is that compatibility with an older version is maintained. This
loses minor GUI functionality (such as setPlaceholderText) but still
allows integration into the window management of the host OS, unlike
when statically linking.

Conflicts:

	doc/release-process.md
Log BerkeleyDB version at startup

Prints the actual version of BerkeleyDB that is linked against, if
wallet support is enabled.

Useful for troubleshooting.

For example:

    2014-05-01 07:44:02 Using BerkeleyDB version Berkeley DB 4.8.30: (April 9, 2010)

    2014-05-01 07:54:25 Using BerkeleyDB version Berkeley DB 5.1.29: (October 25, 20 11)
Update build instructions for Berkeley DB

- People were having problems with the .so when installing in
  alternative locations.
  Like gitian, build a static library with -fPIC that can
  be embedded into the executables.

- Add some missing steps

- Add reminder that BerkeleyDB is only needed when wallet support is
  enabled

Conflicts:

	doc/build-unix.md
qt: add transifex configuration file

This is a project-wide configuration file and should be the same for
everyone.
Also remove mention of creating it yourself from the translation process.
devtools: add a script to fetch and postprocess translations

Run this script from the root of the repository to update all translations from transifex.
It will do the following automatically:

- create a transifex configuration file
- fetch all translations
- post-process them into valid and committable format

Conflicts:

	contrib/devtools/README.md
devtools: add script to check symbols from Linux gitian executables

Add a script to check that the (Linux) executables produced by gitian
only contain allowed gcc, glibc and libstdc++ version symbols.  This
makes sure they are still compatible with the minimum supported Linux
distribution versions.

Conflicts:

	contrib/devtools/README.md
devtools: have symbol check script check for exported symbols

After last commit, our executables should export no symbols anymore.  To
make sure that this stays the case, verify this in the symbol checker
script.
Merge branch '1.7.2-dev-batch3' of github.com:rnicoll/dogecoin into rnicoll-1.7.2-dev-batch3

Conflicts:
	contrib/devtools/README.md
	contrib/devtools/symbol-check.py
	doc/build-unix.md
Merge branch 'rnicoll-1.7.2-dev-batch3' into 1.7.2-dev