Fix CMake error

[Megidd]

Trying to fix a CMake error:

Could NOT find Boost (missing: headers)

Reference: https://stackoverflow.com/q/62782361

[Megidd]

Test

Test shows that after this PR, these statements are fine without any error:

mkdir build
cd build
cmake .. -DLIBNEST2D_HEADER_ONLY=OFF -DCMAKE_INSTALL_PREFIX=./installdir -DCMAKE_TOOLCHAIN_FILE=C:\Users\m3\repos\vcpkg\scripts\buildsystems\vcpkg.cmake
cmake --build . --target install
cmake --build . --target install --config Release