math/igraph: Update to 0.10.6

• Switch to DISTVERSION
• Always use Flex from ports

There are some issues with ARPACK 3.9.0 and igraph,
tracked in PR 273227.

Changelog: https://github.com/igraph/igraph/releases/tag/0.10.6

PR: 273117
Reviewed by: wen (maintainer)
Sponsored by: Blinkinblox