The OLD behaviors of all policies are deprecated, but only by
documentation. Add an explicit deprecation diagnostic for policies
introduced in CMake 3.15 and below to encourage projects to port
away from setting policies to OLD.
7828577065 FindBoost: Add support for Boost 1.71
cf29fa18c8 FindBoost: Unwrap compatibility INTERFACE targets for legacy variables
d7df3cd73f FindBoost: Remove incorrect 1.70 timer dependency
8ff43de1a5 FindBoost: Simplify conditional block for last known version
Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Dennis Klein <d.klein@gsi.de>
Merge-request: !3763
Since commit 0dd6772a89 (FindBoost: Add legacy variables and targets for
compatibility, 2019-06-14, v3.15.0-rc2~3^2~1) we extract information
from imported targets provided by upstream `BoostConfig.cmake` files.
However, upstream Boost 1.71 provides some imported targets only for
compatibility. They are just INTERFACE libraries that wrap around the
real target named by `INTERFACE_LINK_LIBRARIES`. Unwrap this layer so
we can extract the real imported location.
Fixes: #19656
Since commit v3.13.0~7^2 (FindBoost: Improve compiler prefix
detection for GCC 5+ and clang 4+, 2018-11-07) we run the internal
`_Boost_COMPILER_DUMPVERSION` macro on all UNIX platforms. Teach
the macro to tolerate missing `CMAKE_CXX_COMPILER_VERSION`, which
occurs when the CXX language is not enabled.
Fixes: #18624
Add a test for this case to verify the messages. This test will also be
valuable to cover this code path in which we've had several regressions
recently.
