Allow building on Windows under the MINGW64 shell. Multiple changes needed: * pacman command in the build doc was performing a search, not a “sync”, what other package systems call install; thus, no-op * configure was presuming presence of /dev/null meant “not MinGW”; doubtless there are MinGW setups where this is the case, but the modern MinGW64 terminal is not one of these * the guards against trying to use linenoise on Windows weren’t triggered for this type of build, causing link failures (tags: trunk)