SCUMM: Silence C++11 narrowing warnings.
SKY: Silence C++11 narrowing warnings.
AGI: Silence C++11 narrowing warning.
PEGASUS: Silence C++11 narrowing warnings.
SAGA: Silence C++11 narrowing warnings.
SWORD25: Silence C++11 narrowing warnings.
OPENGL: Silence C++11 narrowing warning.
SDL: Silence C++ narrowing warnings.
VIDEO: Silence C++11 narrowing warnings.
AUDIO: Silence C++11 narrowing warnings.
CONFIGURE: Add option to build as C++11.
COMMON: Add a header for C++11 keywords for pre-C++11 compilers.

This currently adds defines for nullptr and override, so we can use them in
pre-C++11 compilers, while taking advantage of them with C++11 compilers.
KYRA: Use override in subclasses of Sound, which override voicePlay.
Merge pull request #301 from lordhoto/c++11-playground

RFC: Allow use of override and nullptr. Also allow C++11 compilation.