Do not exclude include directories made implicit by CPATH
Entries of the CPATH
environment variable are implicitly searched as include directories by some C/C++ compilers. Since !2716 (merged) 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.
Fixes: #19291 (closed)