CMake used to put all header search paths into HEADER_SEARCH_PATHS
attribute. Unfortunately this attribute does not support to declare
a search path as a system include.
As a hack one could add a -isystem /path to the cflags but then include
ordering is not deterministic. A better approach was chosen with this
patch by not filling HEADER_SEARCH_PATHS at all and to populate
the C, C++, and Fortran flags directly. The include paths used by
Xcode should be now identical to the ones used by Unix Makefiles and
Ninja generator.
Direct users of IMPORTED targets treat INTERFACE_INCLUDE_DIRECTORIES
as SYSTEM, after commit a63fcbcb (Always consider includes from IMPORTED
targets to be SYSTEM., 2013-08-29). It was intended that transitive
use of an IMPORTED target would have the same behavior, but that
did not work. The implementation processed only direct dependencies
in cmTarget::FinalizeSystemIncludeDirectories.
Implement transitive evaluation of dependencies by traversing the
link interface of each target in the link implementation.
Unlike other target properties, this does not have a corresponding
non-INTERFACE variant.
This allows propagation of system attribute on include directories
from link dependents.
Gregor Jasny
Stephen Kelly