improve cmake file

CMakeLists.txt

-cmake_minimum_required (VERSION 3.11)
+cmake_minimum_required (VERSION 3.15)
 project (fresnel VERSION 0.1)
 
 set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(default_build_type "Release")
 
-set(PYBIND11_PYTHON_VERSION 3.8)
+#set(PYBIND11_PYTHON_VERSION 3.8)
 
-add_subdirectory (extern/pybind11)
+find_package (Python COMPONENTS Interpreter Development)
+#add_subdirectory (extern/pybind11)
+find_package (pybind11 CONFIG)
 find_package (Eigen3 3.3 REQUIRED NO_MODULE)
-find_package(OpenMP)
+find_package (OpenMP)
 
 pybind11_add_module (_solver source/python.cpp)
 target_link_libraries (_solver PRIVATE Eigen3::Eigen)