gmt5, gmt6: deprecate gmt5, fix sighandler and cmake

[remkos]

Description

• gmt5 and gmt6 Portfiles have been separated as a better forward approach to eventually removing gmt5 (https://trac.macports.org/ticket/71306 and PR gmt5, gmt6: fix common_sighandler patch #26601) as the latter is marked obsolete since the maintainers are already for 5 years on gmt6.
• gmt5 had complications getting compiled on MacOS since the latest revbump (https://trac.macports.org/ticket/71306): this has been fixed with a new patch.
• gmt5 and gmt6 Portfiles have been updated to the latest standards.

This PR thus makes PR #26601 obsolete.
See https://trac.macports.org/ticket/71306

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 15.1 24B83 x86_64
Command Line Tools 16.1.0.0.1.1729049160

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[remkos]

Please note that the gmt6 package passed on all platforms, but not the gmt5 ones.
This is a known issue since #26601 (see also https://trac.macports.org/ticket/71306).

I suggest this PR is accepted so that at least gmt6 works, picking up the sighandler update.
Then some can continue working on gmt5, although we now have marked it obsolete.

[reneeotten]

thanks @remkos!