[PATCH] docs: simplify and clarify the platform support rules

[Daniel P . Berrangé]

The distinction between short life and long life Linux distributions
turned out to be redundant. They can both be covered in a simple way
by noting support will target the current release, and the previous
release for a period of two years or until its EOL. This rule can also
apply to the other UNIX based distros, leaving only Windows needing a
different set of rules.

This also clarifies that Debian LTS is out of scope, because the LTS
support is provided by a separate group from the main Debian maintainer
team.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---

This is a spin off from the Python 3.5 thread

https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg06358.html

 docs/system/build-platforms.rst | 59 ++++++++++-----------------------
 1 file changed, 18 insertions(+), 41 deletions(-)

diff --git a/docs/system/build-platforms.rst b/docs/system/build-platforms.rst
index 9734eba2f1..03d2fd217f 100644
--- a/docs/system/build-platforms.rst
+++ b/docs/system/build-platforms.rst
@@ -29,51 +29,28 @@ The Repology site https://repology.org is a useful resource 
to identify
 currently shipped versions of software in various operating systems,
 though it does not cover all distros listed below.
 
-Linux OS
---------
+Linux OS, macOS, FreeBSD, NetBSD, OpenBSD
+-----------------------------------------
 
-For distributions with frequent, short-lifetime releases, the project
-will aim to support all versions that are not end of life by their
-respective vendors. For the purposes of identifying supported software
-versions, the project will look at Fedora, Ubuntu, and openSUSE distros.
-Other short- lifetime distros will be assumed to ship similar software
-versions.
+The project aims to support the most recent major version at all times. Support
+for the previous major version will be dropped 2 years after the new major
+version is released or when the vendor itself drops support, whichever comes
+first. In this context, third-party efforts to extend the lifetime of a distro
+are not considered, even when they are endorsed by the vendor (eg. Debian LTS).
 
-For distributions with long-lifetime releases, the project will aim to
-support the most recent major version at all times. Support for the
-previous major version will be dropped 2 years after the new major
-version is released, or when it reaches "end of life". For the purposes
-of identifying supported software versions, the project will look at
-RHEL, Debian, Ubuntu LTS, and SLES distros. Other long-lifetime distros
-will be assumed to ship similar software versions.
+For the purposes of identifying supported software versions available on Linux,
+the project will look at CentOS, Debian, Fedora, openSUSE, RHEL, SLES and
+Ubuntu LTS. Other distros will be assumed to ship similar software versions.
 
-Windows
--------
-
-The project supports building with current versions of the MinGW
-toolchain, hosted on Linux.
-
-macOS
------
-
-The project supports building with the two most recent versions of
-macOS, with the current Homebrew package set available.
+For FreeBSD, decisions will be made based on the contents of the ports tree;
+for macOS, `HomeBrew`_ will be used, although `MacPorts`_ is expected to carry
+similar versions.
 
-FreeBSD
+Windows
 -------
 
-The project aims to support all versions which are not end of
-life.
-
-NetBSD
-------
-
-The project aims to support the most recent major version at all times.
-Support for the previous major version will be dropped 2 years after the
-new major version is released.
-
-OpenBSD
--------
+The project supports building with current versions of the MinGW toolchain,
+hosted on Linux (Debian/Fedora).
 
-The project aims to support all versions which are not end of
-life.
+The version of the Windows API that's currently targeted is Vista / Server
+2008.
-- 
2.26.2