Fix for SDL3 include autoconf

[rakslice]

Fix so that the autoconf modification of the SDL3 include path works on systems where the default shell is not bash (or other shell that has this += operator)

[kanjitalk755]

Could you also include BasiliskII/src/Windows/configure.ac and SheepShaver/src/{Unix,Windows}/configure.ac in this PR?

[rakslice]

I made the change also for BasiliskII/src/Windows/configure.ac and SheepShaver/src/{Unix,Windows}/configure.ac.

For Windows it should not really be necessary since the build environment is always MSYS2?

What is the situation with SDL3 support in Windows? I see MSYS2 doesn't have a packaged sdl3 for mingw32. Do you just build it from source?

[kanjitalk755]

For Windows it should not really be necessary since the build environment is always MSYS2?

Ah, it may not be necessary for MSYS2.

What is the situation with SDL3 support in Windows? I see MSYS2 doesn't have a packaged sdl3 for mingw32. Do you just build it from source?

I was building SDL3 from source.
The official release (3.2.0) came out last week, so I think it will be supported in MSYS2 as well.