cmake: Fix usage of CMAKE_DEPENDENT_OPTION

CMAKE_DEPENDENT_OPTION takes a value argument, but as a macro function
it will read a variable name as the name and not the value. For
YUZU_USE_BUNDLED_QT, ensure that we are reading the value of MSVC. For
YUZU_ALLOW_SYSTEM_SDL2, CMAKE_DEPENDENT_OPTION is redundant here anyway
as we don't use that path on any toolchain by default.