gptfdisk: add support for building and install cgdisk

In addition to the command-line tool gdisk and the interactive tool
sgdisk, gptfdisk also provides a ncurses based tool, cgdisk. This
commit adds support for building and installing this tool.

[Peter: Update gdisk autoselection logic to consider cgdisk as well]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>