[CMake] target_include_directories(): SYSTEM option does not seem to work on recent versions of CMake, when targeting Xcode.

[A D]

Hi,

  We are moving our codebase from native project file to CMake based
management. The process is going well, but we need the Boost dependency to
be configured as a system library (to avoid spamming the build log with
Boost related warnings).

It is our understanding that giving a SYSTEM argument to
target_include_directories function should do just that. Yet it does not
seem to work as expected. We created a minimal example that reproduces the
problem:

cmake_minimum_required(VERSION 3.0)

find_package(Boost 1.49 COMPONENTS)

project(system_dependencies)

add_executable(${PROJECT_NAME} main.cpp)

target_include_directories(${PROJECT_NAME} SYSTEM PUBLIC ${Boost_INCLUDE_DIRS})


The compiler invocation issued by Xcode contains this Boost header search
path:
*-I/Users/.../SDK/boost/include*
It is not using "-isystem" flag, whereas it is available.

Did we misuse the command ? Or should we report this as a bug ?

Thank you for reading,


----

The environment:
CMake v3.3.0 (previously tested with v3.0.0 for the same results)
OS X 10.9.5
Xcode 5.1.1

There is a Stack overflow question opened for this problem:
http://stackoverflow.com/q/31722426/1027706
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