app-emulation/sheepshaver: fix SDL version selection

[living180]

On my system passing --without-sdl1 when configuring sheepshaver actually causes it to look for SDL1 and ignore SDL2. sheepshaver actually defaults to SDL2, so specifying --without-sdl1 should be redundant, and removing it fixes the build for me.

[Tatsh]

What do you have installed that still wants SDL 1?

[living180]

So I should have provided more detail in my initial description. Here's what I'm seeing:

• On my system I have only media-libs/libsdl2 installed (no SDL 1)
• If --without-sdl1 is passed to ./configure for sheepsaver, on my system it actually looks for SDL 1 and doesn't find it, because it isn't there. It does not look for SDL 2. It then fails to compile because the include path for SDL is not passed to the compiler (I did a bit of digging in the configure scripts for Sheepsaver to try to figure out why this is happening, but was not able to get very far.)
• If I do not pass --without-sdl1 to ./configure, sheepsaver by default looks for SDL 2, finds it, and is able to build successfully.

Hopefully this makes what is happening on my system a bit more clear.