Digikam 5.2.0 is out with a lot of fixes for the 5.x series https://www.digikam.org/node/758
By simple bump the ebuild version and remove the patches section in the 5.1.0 build file, I successfully build it with -handbook, and now without any issues of using it. But just curious why it can't be build with USE flag of handbook.
Thanks, fixed in git. Sorry for the delay, this is quite a tricky package to maintain and could use some love from someone who actively tracks upstream. https://gitweb.gentoo.org/repo/gentoo.git/log/?qt=range&q=d9659b1a6374be3a1fc2e19c4ae3e25e6883a0ef...0447805aae8668a280c3f67a9da8b994837f687c