src/aboutdlg.cc: Use string::erase() rather than assigning the
result of string::substr().
src/: Use cast to void rather than assignment to self to suppress
unused parameter warnings, as clang warns about the latter.
src/moviemaker.h: If we don't have libav or ffmpeg, omit all the
member variables from class MovieMaker, to avoid "unused private
field" warnings from clang.
src/printwx.cc: Include wx headers before ISO C/C++ ones, to try to
fix build failure on OS X 10.9 with wx 3.0.0.