meson.build: Fix disabling fontconfig by default on windows

Setting a 'require' option on a dependency to false doesn't actually disable it, so e.g. wraps still get pulled if enabled.

With a newer meson version, the solution is feature.disable_auto_if() but that's not available until 0.59.0. In the mean time, manually write a condition that sets fontconfig_dep to a not found dependency in the conditions where it should be disabled.