CMake: allow use of system pybind11 & install fatropy

[nim65s]

Hi,

Here is a little packaging update for python users.

[maximvochten]

@nim65s

Thanks for your contribution. For me however the installation fails now on this line:

 find_package(Python REQUIRED COMPONENTS Interpreter Development.Module)

It results in the following error:

  CMake Error at /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:146 (message):
    Could NOT find Python (missing: Development.Module) (found version
    "2.7.18")

If I explicitly enforce Python3 using

 find_package(Python3 REQUIRED COMPONENTS Interpreter Development.Module)

it still results in an error:

  CMake Error at /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:146 (message):
    Could NOT find Python3 (missing: Development.Module) (found version
    "3.8.10")

Python3-dev is installed with

  sudo apt-get install python3-dev

I think there is a confusion between different versions of Python.
Any idea how to solve this?

[nim65s]

Thanks for the ping, but I think we can continue in #20