Allow use of system pybind11

[ghisvail]

In case pybind11 is not available through find_package, fall back to the previous behavior.

[ghisvail]

Seems like the use of pybind11_DIR to point to the vendored versions is colliding with the find_package mechanics.

Best would be to use pybind11_SOURCE_DIR as the variable name instead, and let CMake do its thing with pybind11_DIR like it does for ITK_DIR.

I will propose a better patch in that direction.