Move logic out of CMakeLists.txt

This is an initial pass, simply moving the existing logic as-is. Future
PR's will attempt to refactor and improve the cmake stuff.

cMake/FreeCAD_Helpers/SetupPybind11.cmake

@@ -0,0 +1,10 @@
+macro(SetupPybind11)
+# -------------------------------- PyBind11 -----------------------------
+
+    # necessary for flat-mesh feature
+    option(FREECAD_USE_PYBIND11 "Use pybind11" OFF)
+    if (FREECAD_USE_PYBIND11)
+        find_package(pybind11 REQUIRED)
+    endif()
+
+endmacro(SetupPybind11)