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Entries of the `CPATH` environment variable are implicitly searched as
include directories by some C/C++ compilers. Since commit 5990ecb741
(Compute implicit include directories from compiler output, 2018-12-07,
v3.14.0-rc1~108^2) these entries are detected by CMake and included in
the `CMAKE_{C,CXX}_IMPLICIT_INCLUDE_DIRECTORIES` variables.
However, we should not exclude them from explicit specification via `-I`
or particularly `-isystem` because they are meant as user-specified
include directories that can be re-ordered without breaking compiler
builtin headers. In particular, we need explicit requests via
`include_directories` with the `SYSTEM` option to result in `-isystem`
so that third-party headers do not produce warnings.
Co-Author: Ben Boeckel <ben.boeckel@kitware.com>
Fixes: #19291
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