configure.in additions for Emscripten

Now ./configure will detect when emcc is being used, and set
up things for Emscripten. Auto-detection can be overridden via
--enable-emscripten. Comments explain the changes.
Disable SDL CD and CD image support on Emscripten.

Emscripten SDL doesn't have SDL CD support. The CD image support
relies on a SDL Mutex, which is not supported by Emscripten.
Disable overlay screen on Emscripten.

Emscripten doesn't support the SDL YUV overlay functions.
Compile error fixes

This finally makes it possible to compile DOSBox with Emscripten.
This is not a functional state!
Simple but ugly hack to make DOSBox usable in a web browser.

This runs the machine normally once, allowing mount to be run
to gain access to files. The second run causes Normal_Loop() to
become the Emscripten main loop. The command prompt cannot be
used interactively.

To use DOSBox, make a DOS executable accessible, for example
via --preload-file while building. When running, set arguments
to just that file, in the current directory. For example, add
"arguments: [ './filename.exe' ]" to Module in the HTML file.
TOTAL_MEMORY also needs to be increased. 128 MB always works.
Removing -g when linking creates a much smaller JavaScript file,
and greatly reduces Firefox memory usage.
Fix keys in Emscripten by not using scancodes.

Keys are very mixed up if using scancodes, and scancode support
doesn't seem worthwhile because Emscripten generates them from
SDL keys.