[th/meson-auto-ifcfg-rh] build/meson: fix autodetecting whether to enable ifcfg_rh/ifupdown plugins

"-Difcfg_rh=false" did not work, we would always fallback to autodetection. That is wrong, an explicit true/false should be honored. It's also not what autotools does. Fix this.

While at it, drop "distro". It's unused otherwise.

Also, no longer let the autodetection be based on cross compilation. When cross-compiling, it seems not entirely unreasonable that you cross compile to a comparable distro, so let the autodetection be based on what we detect on the host. In any case, a user can and is encouraged to explicitly enable/disable the plugins via "-Difcfg_rh=" or "-Difupdown=".