Searches for SDL* even when ALSOFT_BACKEND_SDL* is OFF

[ryandesign]

cmake searches for SDL (with find_package) even if ALSOFT_BACKEND_SDL2 is OFF:

openal-soft/CMakeLists.txt

Lines 101 to 102 in 6790234

	find_package(SDL3 QUIET)
	find_package(SDL2 QUIET)

I can counteract this by using -DCMAKE_DISABLE_FIND_PACKAGE_SDL2=ON but I don't think I should have to do that. For all the other backends, the corresponding find_package call is not made if the backend is disabled.

[kcat]

Commit 5c17dfd avoids searching for SDL2 when the backend is disabled. SDL3 still gets searched for since it's needed for a couple examples.