libpst: update to 0.6.76

[srirangav]

Description

Update libpst to version 0.6.76.

Closes: https://trac.macports.org/ticket/63060

Type(s)

• bugfix
• enhancement
• security fix
• [ X] update

Tested on

macOS 11.4

Verification

Have you

• [ X] followed our Commit Message Guidelines?
• [ X] squashed and minimized your commits?
• [X ] checked that there aren't other open pull requests for the same change?
• [X ] referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[ryandesign]

CI originally failed because I had made a change to gimp2 that required MacPorts 2.7 or newer. I knew our CI was still running MacPorts 2.6.4 but I did not anticipate that changing gimp2 would affect other ports. I fixed gimp2, rebased this PR branch, and force-pushed.

CI then succeeded for 10.14 so I will merge. The build on macOS 11 failed apparently because our CDN returned an unexpected error when trying to download a package. I'll ask our CDN support why. The 10.15 build says it was cancelled; I don't know why or by whom.