meson: skip SDL2 detection if --disable-system

SDL is only used for system emulation; avoid spurious warnings for
static --disable-system emulation by skipping the detection of
the library if there are no system emulation targets.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed