Use feature options for things that are optional features,
update the docs.
Visible changes here is that the 'print-backends' option
got renamed to 'print' to go better with 'media', and the
'tracker3' option got renamed to 'tracker'.
For options that have been changed into features, the
syntax now is -Dfeature=enabled or -Dfeature=disabled
or -Dfeature=auto.
The win toolchain has problems linking meson subprojects to libgtk.
To work around that build glib/pango from git and install them first
as long as the installed version is too old at least.
Fixes#3002
They use powershell instead of cmd.exe, use the tag win32-ps instead of win32,
and run Windows 2016 instead of 2012r2.
The old runners will be switched off in the comming weeks.
A recent dependency change in MSYS2 made it pull in vulkan, which made
meson think it's available but it somehow links against the system vulkan dll
instead.
Disable vulkan for now.
There where some problems (??) with ccache not detecting changes during meson
checks. Setting CCACHE_DISABLE during the meson execution makes ccache not use
the cache and pass things directly to the compiler.
