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.
qtcreator: Update to 2.8.1
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).
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.