Add support for building `emux' on Mac OS X.

This is the first implementation of Mac OS X build for `emux'.
It has the following impacts:
  * include files that place structures in non-default sections
    must specify the memory segment as well (__DATA).
  * modules define a constructor responsible for retrieving sections
    boundaries from the Mach-O file at startup and storing them in
    $module_{begin,end} variables.
  * sections boundaries variables must be declared with global scope
    visibility (i.e. not `static') for use in `main' module (`print
    usage' use case).
  * configure/make does not pass linker script parameter to gcc if
    building on Mac OS X.

The following triggers are used to enable this behavior:
  * in source code: __APPLE__ macro is defined
  * in autotools: target system is `darwin'-ish

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>
Fix SDL video frontend on Mac OS X

On Mac OS X, header file `SDL.h' must be included before main() function
is defined, so that its name is substituted with SDL_main().
Only perform the inclusion for that target, and if emux was compiled with
`SDL' support.

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>