BUILD: Use LTLIBICONV instead of LIBICONV

We use libtool, after all.
MAIN: Change the WinMain() stuff for SDL2

SDL1.2 had you call SDL_SetModuleHandle(), which SDL2 doesn't.
SDL2 does need SDL_MAIN_HANDLED, though, since we define our
own WinMain().
BUILD: Put libs into xoreos_LDADD instead of AM_LDFLAGS

This should preserve the correct linking order for statically
linking, i.e. put the all external libraries after our internal
convenience libraries.