[CMake] relative path to toolchain does not work

[hex]

I specified my toolchain like so:

1|cmake -DCMAKE_TOOLCHAIN_FILE=myToolchain.cmake .. |//

and created a toolchain file in|project/myToolchain.cmake|.

Using relative path CMake first looks relative to the top of the build 
directory, then if not found there, relative to the top of the source 
directory.

What means “the top of the source directory”? I placed the file in the 
source directory and receive:

1234|CMake Warning: Manually-specified variables were not used by the 
project: CMAKE_TOOLCHAIN_FILE|

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