games/exult: fixup after aae00cbee1cef8d51abff5de932f7cf64c62dc87

Apparently, I did not pick up the very last patch in the PR.

This additional commit adds the remaining bits discussed in the PR:

• Convert to DISTVERSION
• Use c++17
• Add missing glib dependency
• Make ALSA optional
• Reorder some variables

PR: 254801
Reported by: gspurki@gmail.com