Using platforms in 2.8.0

[Joshua Root]

MacPorts 2.8.0 lets you specify which OS versions your ports work on via 
the platforms option. This indicates a port that works on darwin 
versions from 10.x to 19.x inclusive:

platforms {darwin >= 10 < 20}

Most ports will probably only need one comparison, but you can have as 
many as you need. Operators supported are ==, !=, >, <, >= and <=. For 
== and !=, a string comparison that allows globs is used. For the rest, 
vercmp is used. If any comparison doesn't match, known_fail is set to yes.

Note that the comparison is against ${os.version}, not ${os.major}, so 
you'll usually want to use e.g. < 20 for the upper bound rather than <= 
19 (the latter would exclude all 19.x versions).

These are all valid:

platforms {darwin >= 11}
platforms {darwin < 19} freebsd
platforms {darwin >= 16 != 18.2.* < 23} {linux != *}

The second one indicates FreeBSD support in a purely informational way, 
as per the usage of the platforms option in older MacPorts versions. The 
last one indicates that it doesn't work on Linux and will set known_fail 
there. Again, most ports will probably only use something like the first 
example, but the flexibility is there if you need it.

All of the above can be used without making your Portfile incompatible 
with 2.7. There is another way that platforms can be used:

platforms any
platforms {darwin any}

The first one indicates that the port will install identical files no 
matter what platform it is built on, and will set the platform in the 
archive filename to "any_any". The second one indicates that the port 
will install identical files when built on any version of Darwin, but 
may install different files when built on other platforms, and sets the 
platform in the archive filename to "darwin_any" when on Darwin.

These will usually only be applicable to noarch ports, though rare 
exceptions may exist. Ports that install only data files or scripts will 
often be able to use "any". Python scripts are an exception because 
Python uses a framework layout on Darwin only, so they will be "{darwin 
any}".

Since the archive filename is changed, using this will of course prevent 
users on older MacPorts versions from using the archives. So we would 
normally wait 2 weeks after the release of 2.8.0 before starting to use 
this feature.

However, with the release of Ventura in a couple days, it may pay to set 
these platforms values on as many eligible ports as possible, both to 
make the archives available to Ventura users even before we have a 
Ventura buildbot worker up and running, and to reduce the load on that 
worker once it is up.

- Josh