Disable exec symlink resolution for PostgreSQL 9.2

When building PostgreSQL with plugins under NixOS, NixOS will create a
postgresql-and-plugins directory which symlinks PostgreSQL and all the plugins
into a single directory. Unfortunately, the plugins will not actually be usable
by PostgreSQL because it will still try and locate them in the original
PostgreSQL share directory, not postgresql-and-plugins.

In this commit, I have patched resolve_symlinks to always return success, which
matches the behavior if HAVE_READLINK is false (so presumably invalid paths are
never passed to this function).
qtcreator: Update to 2.8.1
nixos/nginx: allow to specify which package to use

This enables to override which plugins to include with nginx by allowing to change
nginx package. I also removed webdav option in nginx nixos module, because you
can now specify this by overriding nginx package.
qtcreator: Make the qt package optional and introduce the QtSDK

To give the ability to use a different Qt version than the default one
(which can build 3 different times Qt Libraries if we mixed the default
one, the qtcreator one and the version including all the examples and the
docs).

Right now a developer can choose to directly install the QtSDK which
includes a "full" (developerBuild + docs + examples) Qt version and uses
it to build QtCreator.

The possibility to only install QtCreator and its previous behavior has
been kept for flexibility purposes (we do not need to force someone on the
SDK approach).
mesa: Minor fix on GL/gl.h

Apply a fix which prevented to use -DGL_GLEXT_LEGACY, -Werror and -Wundef
to be used together. This produced a build fail on any software meeting
these requirements.
Add Qt 5.1.1 and its SDK

Consider this as a first step towards the integration of Qt5 into nixpkgs,
it does not yet intends to replace Qt4 on every packages even if possible.
My goal here is to have a first derivation in common between people who
needs qt5 for development purposes.

The derivation has been written from scratch but I took care to read at the
version 4 to re-integrate some patches which are still compatible. However,
I did not had enough time to test gtkStyle and flashplayerFix as I do not
use any of them. Also, OSX users will have to do some extra work because
I do not have any mac.

Finally, as some configure flags have changed and in an hope to provide a
clear package definition before it becomes mature, I voluntary added some
flags which are default. Once every option will be mastered, we will just
have to redo a pass on qt5 configure flags and remove the ones which are
set by default.
vtk: Use the Qt libraries as an argument instead of a boolean

To prevent multiple Qt libraries when developing with a custom one, the Qt
support can now be activated by directly supplying the Qt libraries as an
argument (qtLib).

qtSDK and qtFull users/developers now just have to define an override such
as the following one in order to use it inside their development
environment:

vtk.override { qtLib = qt4SDK; };

The previous behavior is still the same for vtk and vtkWithQt4 end-users.

Change-Id: I517762d4ff7de46d32cc46e6e725fd62737caa52
Attempt at compiling MilkyTracker, but recent zlib version may have broken it.

There's a zlib version included with milkytracker,
but there's no makefiles for it. I've only included
the header here, but it fails at link-time with
several 'undefined reference' errors, which simply
means it can't find the definitions, e.g. compiled
zlib.
There's bug reports on other package systems although
unfortunately still unresolved.
https://bugs.archlinux.org/task/31324
http://lists.freebsd.org/pipermail/freebsd-ports/2013-March/082180.html
MilkyTracker: fix build, use system zlib and decompressor patch

Also add JACK support
MilkyTracker: Tidy up, sort buildInputs and use the attrPath for the license
Set the channel for this release
nixos/logstash: update and simplify to be fully compatible with new version
nixos/logstash: add option examples
nixos/logstash: add test
fetchsvn: handle redirect(s)

Instead of failing if the server redirects us to a new URL, accept up to
2 redirects by printing 2 extra p's to subversion.
haskell-wxc: noHaddock
venus.nix: Fix broken startAt option
Make touchpad button mapping configurable
mathematica: fix x86 build of mathematica

the MathInstaller doesn't distinguish between 64bit and 32bit linux
platforms.
quodlibet: update to version 2.6.3

also provides two distinctive flavours, which either wrap the program to
pull in the gstreamer plugins or not.