Arch-indep CMake packaging (#764)

* Arch-indep CMake packaging

Since pybind11 is a header-only library, the CMake packaging does not have to carry any architecture specific checks. Without this patch, the detection of pybind11 will fail on 32-bit architectures if the project was built on a 64-bit machine and vice-versa. This fix is similar to what is applied to `Eigen` and other header-only C++ libraries.