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.
