build: don't mandate availability of a fetcher program

build: don't mandate availability of a fetcher program

It is common that build hosts are isolated from outside world. They
don't necessarily have wget or ftp installed.

Turn the error into warning in configure. And point FETCHER to `false'
command if neither wget nor ftp is available, so any attempt to
download will result in error.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>