Disable -Wunused-function for stb_image.h

Unfortunately it doesn't work for gcc, the only way to stop these warnings in
gcc is to use attributes. But since stb_image.h is a third party dependency,
modifying its code is discouraged.
Silence unused function warning in glfw portion of the demo app.

This function needs to be defined only starting from a certain version of the
GLFW, where it can actually be used. It's the drag'n'drop functionality.
Enable -Wall -Wextra -Wno-unused-parameter on gcc and clang. Fixes #23.

At the moment gcc still generates a bunch of warnings on stb_image.h, but we
can't workaround that easily without modifying the stb_image.h itself, which
is a third party code.
Merge pull request #27 from nsf/master

Enable -Wall -Wextra -Wno-unused-parameter on gcc and clang. Fixes #23.