Modernises the cflow formula with SSL/TLS links, SHA256, double quotes
everywhere, etc. Also added a test, which I completely and utterly
stole from the GNU manpage for cflow.
ClosesHomebrew/homebrew#37565.
Signed-off-by: Brett Koonce <koonce@gmail.com>
It seems that the main GNU download site has issues in some places
outside the U.S., so we'll use the provided "ftpmirror.gnu.org" to pick
a nearby mirror.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>