configure: quote -D options that are passed through to meson

configure: quote -D options that are passed through to meson

Ensure that they go through unmodified, instead of removing one layer
of quoting.

-D is a pretty specialized option and most options that can have spaces
do not need it (for example, c_args is covered by --extra-cflags).
Therefore it's unlikely that this causes actual trouble.  However,
a somewhat realistic failure case would be with -Dpkg_config_path
and a pkg-config directory that contains spaces.

Cc: qemu-stable@nongnu.org
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>