It's a bad thing to start an answer with a question, but I do it nevertheless:
- First, am I right in assuming that you've set an environment variable BOOST_ROOT pointing to C:\Boost? (Simply adding it to the path might not be sufficient here.)
- Second, which CMake generator are you running? Visual Studio, Makefile, ninja?
I've head a similar problem recently that was connected to the generator that I was actually using. More precisely, I was trying to create a Ninja project from within cmake-gui and got almost the same error message. However, I was able to create a Visual Studio project in cmake-gui project without problem.
When looking a little closer at the command output for the Ninja case I found the following two lines at the very beginning:
The C compiler identification is unknown
The CXX compiler identification is unknown
This hints at the actual problem. While it is clear which compiler will be in use for Visual Studio (9, 10, 11, ...), cmake cannot infer a default compiler for Ninja because it is a generic build system that runs with different compilers. In the end boost wasn't found because the compiler is unknown.
A simple solution was to open the Developer Command Prompt for the Visual Studio version that you want to run with. When you create the project in this "extended" command prompt CMake will be able to infer the correct compiler. Alternatively, you can set the CMAKE_CXX_COMPILER and CMAKE_C_COMPILER flags when running cmake.
cmake -G "Ninja" <path/to/CMakeLists.txt>
cmake -G "Ninja" <path/to/CMakeLists.txt> -DCMAKE_CXX_COMPILER="path/to/cxx/compiler" -CMAKE_C_COMPILER="path/to/c/compiler"