meson: Handle the b_lto option as a string option for newer meson versions
meson: Handle the b_lto option as a string option for newer meson versions

Since 3e6fbde94c1cb8d4e01b7daf0282c315ff0e6c7d in meson (past the
0.56 release), the b_lto option was changed from a bool to a
tristate option (false/true/thin).

One could just compare the b_lto option against 'false', but that
causes warnings on older meson versions (on all existing releases).

videolan/dav1d