configure: Auto-detect and enable C++11 support if available

Enabling support for C++11 adds CONFIG+=c++11 to the Qt build. Projects
using Qt can check for C++11 support using contains(QT_CONFIG, c++11) in
their .pr[iof] files.

The QMAKE_CXXFLAGS_CXX11 and QMAKE_LFLAGS_CXX11 qmake varibles contain
any arguments the compiler needs to enable C++11. CONFIG+=c++11 adds
these arguments to the build.

Support for clang, g++, and the Intel C++ Compiler for Linux are
included in this commit.

Change-Id: Id77f86d7ad4d5c740b890446a40b105879a0d327
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>