This mimics CMake's policy support. The policy state is stored in an
internal __QT_INTERNAL_POLICY_<policy> variable; by using normal
variables, we gain support for stacking for free.
Policies can be explicitly en- or disable via qt6_policy; that command
also has a GET mode to retrieve them again.
Furthermore, one can now pass min and max version to
qt6_standard_project_setup, to opt in to a certain set of defaults
introduced in a given Qt version.
We add support for policies in QtModuleHelpers, so that we can check for
known policies while building Qt itself.
No actual policies exist yet; but a follow up commit will introduce one
for qt_add_qml_module.
Change-Id: I57a0404c9193926dd499f94cc5f73e359355c0b3
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
b9c80ecc08