Consider to work around GCC Bug 58800 (std::nth_element segfaults)

...at least for users building Qt from source with an unpatched/vanilla GCC/libstc++ 4.8.1/4.8.2.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58800

The bug affects clients of std::nth_element and might lead to a crash. qtbase/src/concurrent/qtconcurrentmedian.h seems to be the only client so far. Prominent crashing application is Qt Creator when executing "Find Usages": QTCREATORBUG-10565.

According to that bug report the GCC bug is fixed for versions "4.7.4/4.8.3/4.9.0.", but not for the last two released GCC versions: 4.8.2 and 4.8.1.

Allan Sandfeld Jensen proposed in QTCREATORBUG-10565 to #ifdef out the use of nth_element for the affected versions of libstdc++. This will only affect about 3 lines since the commit introducing nth_element is quite trivial: 880b614c8ffd530251b9383e085ba094a1edbca8.

Further notes:

• Currently Ubuntu 13.10 provides the affected GCC 4.8.1, though they have a patched version in the pipe line: https://bugs.launchpad.net/ubuntu/+source/gcc-4.8/+bug/1246802