$ cat ../CMakeLists.txt
set(CMAKE_TOOLCHAIN_FILE ${CMAKE_CURRENT_SOURCE_DIR}/toolchain.cmake)
cmake_minimum_required(VERSION 2.8)
project(Issue13796 C)
$ cat ../toolchain.cmake
message(WARNING "CMAKE_TOOLCHAIN_FILE=[${CMAKE_TOOLCHAIN_FILE}]")
$ cmake --version
cmake version 2.8.10.2
$ cmake ..
CMake Warning at toolchain.cmake:1 (message):
  CMAKE_TOOLCHAIN_FILE=[.../toolchain.cmake]
Call Stack (most recent call first):
  .../share/cmake-2.8/Modules/CMakeDetermineSystem.cmake:89 (include)
  CMakeLists.txt:3 (project)

CMake Warning at toolchain.cmake:1 (message):
  CMAKE_TOOLCHAIN_FILE=[.../toolchain.cmake]
Call Stack (most recent call first):
  b/CMakeFiles/2.8.10.2/CMakeSystem.cmake:1 (include)
  CMakeLists.txt:3 (project)

-- The C compiler identification is GNU 4.7.1
-- Check for working C compiler: /usr/bin/cc
CMake Warning at .../toolchain.cmake:1 (message):
  CMAKE_TOOLCHAIN_FILE=[]
Call Stack (most recent call first):
  .../b/CMakeFiles/2.8.10.2/CMakeSystem.cmake:1 (include)
  CMakeLists.txt:2 (PROJECT)

-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
CMake Warning at .../toolchain.cmake:1 (message):
  CMAKE_TOOLCHAIN_FILE=[]
Call Stack (most recent call first):
  .../b/CMakeFiles/2.8.10.2/CMakeSystem.cmake:1 (include)
  CMakeLists.txt:2 (PROJECT)

-- Detecting C compiler ABI info - done
-- Configuring done
-- Generating done
-- Build files have been written to: .../b