Setting SDK on cross-compilation to OSX in CMake - macos

Question

I'm cross compiling from Linux to OSX using CMake. To do this I use a toolchain file, so the call is like this:
cmake -G 'Unix Makefiles' -DCMAKE_TOOLCHAIN_FILE=./cmake/toolchains/c.apple.universal.cmake .
The toolchain file looks like this:
SET(CMAKE_SYSTEM_NAME Darwin)
SET(CMAKE_SYSTEM_PROCESSOR universal)
# set compilers...
INCLUDE("${CMAKE_CURRENT_LIST_DIR}/c.apple.common.cmake")
And the other one:
INCLUDE("${CMAKE_CURRENT_LIST_DIR}/../Modules/CMakeMacroSetCCache.cmake")
# specify the cross compiler
SET_CCACHE(CMAKE_C_COMPILER i686-apple-darwin10-gcc)
SET_CCACHE(CMAKE_CXX_COMPILER i686-apple-darwin10-g++)
SET(CMAKE_RANLIB i686-apple-darwin10-ranlib CACHE STRING "" FORCE)
SET(CMAKE_LIPO i686-apple-darwin10-lipo CACHE STRING "" FORCE)
SET(OSX104_SDK "/usr/lib/apple/SDKs/MacOSX10.4.sdk")
SET(OSX105_SDK "/usr/lib/apple/SDKs/MacOSX10.5.sdk")
# set SDK
SET(CMAKE_OSX_DEPLOYMENT_TARGET )
IF(EXISTS ${OSX104_SDK})
SET(CMAKE_OSX_SYSROOT ${OSX104_SDK})
ELSEIF(EXISTS ${OSX105_SDK})
SET(CMAKE_OSX_SYSROOT ${OSX105_SDK})
ELSE()
MESSAGE(FATAL_ERROR "No OSX SDK found!")
ENDIF()
MESSAGE(STATUS "Using OSX SDK at ${CMAKE_OSX_SYSROOT}")
SET(CMAKE_PREFIX_PATH ${CMAKE_OSX_SYSROOT})
SET(CMAKE_FIND_ROOT_PATH ${CMAKE_PREFIX_PATH})
SET(BOOST_ROOT ${CMAKE_PREFIX_PATH})
# search for programs in the build host directories
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
# for libraries and headers in the target directories
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
This works so far, but somehow CMake resets CMAKE_OSX_SYSROOT at some point. The output I get is:
-- Set CMAKE_C_COMPILER to /usr/lib/ccache-lipo/i686-apple-darwin10-gcc
-- Set CMAKE_CXX_COMPILER to /usr/lib/ccache-lipo/i686-apple-darwin10-g++
-- Using OSX SDK at /usr/lib/apple/SDKs/MacOSX10.5.sdk
-- Set CMAKE_C_COMPILER to /usr/lib/ccache-lipo/i686-apple-darwin10-gcc
-- Set CMAKE_CXX_COMPILER to /usr/lib/ccache-lipo/i686-apple-darwin10-g++
-- Using OSX SDK at /usr/lib/apple/SDKs/MacOSX10.5.sdk
-- The C compiler identification is GNU
-- The CXX compiler identification is GNU
-- Checking whether C compiler has -isysroot
-- Checking whether C compiler has -isysroot - yes
-- Checking whether C compiler supports OSX deployment target flag
-- Checking whether C compiler supports OSX deployment target flag - yes
-- Check for working C compiler: /usr/lib/ccache-lipo/i686-apple-darwin10-gcc
-- Check for working C compiler: /usr/lib/ccache-lipo/i686-apple-darwin10-gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Check for working CXX compiler: /usr/lib/ccache-lipo/i686-apple-darwin10-g++
-- Check for working CXX compiler: /usr/lib/ccache-lipo/i686-apple-darwin10-g++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Used Toolchain definition file '/srv/jenkins/.../cmake/toolchains/c.apple.universal.cmake'
-- Configuring for cross-compiling to Darwin on universal
-- Using platform config cmake/darwin.cmake
-- Checking /Developer/SDKs/MacOSX.sdk/usr/lib/libSystem.B.dylib for possible architectures
The last 3 lines are from the CMakeList.txt. The correct toolchain file was used (it's just a message), SYSTEM_NAME and SYSTEM_PROCESSOR are set correctly but the OSX SDK is wrong. The corresponding CMake code comes from a macro that will set CMAKE_OSX_ARCHITECTURES if it wasn't set before (which is the case here). The line is:
MESSAGE(STATUS "Checking ${CMAKE_OSX_SYSROOT}/usr/lib/libSystem.B.dylib for possible architectures")
Am I using CMake wrongly here? Why is CMake resetting the OSX_SYSROOT? According to http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_OSX_SYSROOT.html it should also affect FIND* commands but I needed to set CMAKE_FIND_ROOT_PATH for it to work.
Strange enough, it worked yesterday. Cleaning the whole build-directory (which I'm sure I did yesterday too) and rerunning CMake resets CMAKE_OSX_SYSROOT now.
If this makes a difference: PROJECT is called before the inclusion end execution of the macro, that shows the wrong sysroot.

Answer 1

The solution I found is to set the variable into the cache. This way it "survives" during CMake processing:
SET(CMAKE_OSX_SYSROOT ${CMAKE_OSX_SYSROOT} CACHE PATH "..." FORCE)