gptfdisk: make utf16 support configurable

Instead of always disabling the UTF16 support, make it
configurable. When ICU is available, enable UTF16 support, otherwise
disable it.

[Peter: wrap long line]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>