Compilation with GCC 4.6

[Christoph Feck]

Hi,

I recently switched to GCC 4.6.0, and compiled (nearly) all of svn trunk resp. 
git master. Everything went fine, except for two issues:

* Eigen2 uses ptrdiff_t, but does not include <cstddef>, causing a compilation 
error in "calligra" and "kstars". This bug has been reported to Eigen 
developers[1], so the question is, if we should just ignore the bug, in other 
words, "if you use a newer gcc, make sure you try to find a newer Eigen" or if 
we should help here and add a few temporary include lines; I have local 
patches for calligra and kstars adding those.

* There is an annoying[2] warning in 
kdelibs/kdecore/config/conversion_check.h about unused variable x in lines 
53 and 60. It looks like the Q_UNUSED needs to be moved to the template 
function where the variable is first defined, right? Proposed patch on RB[3].

A third issue in playground/games has been reported to the respective author.

Generally, I can only recommend GCC 4.6.0; it issues some additional 
warnings and also produces better code for certain constructs, while being a 
bit faster at it, too.

Christoph Feck (kdepepo)

[1] http://eigen.tuxfamily.org/bz/show_bug.cgi?id=250
[2] It is only annoying because it shows up nearly everywhere
[3] https://git.reviewboard.kde.org/r/101138/