Move to meson the code to detect the presence of accelerators, and
to define accelerator-specific config-target.h symbols.

The logic for now is duplicated in configure because it is still
in use to build the list of targets (which is in turn used to
create the config-target.mak files).  The next patches remove it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>