Use CMake's own FindPythonLibs instead of vendored FindPythonLibsNew.

This enables easy cross-compilation and pybind11 doesn't have to carry
around an old version of FindPythonLibs.

Author: Cody Piersall

CMakeLists.txt

@@ -137,7 +137,6 @@ if (PYBIND11_INSTALL)
   set(CMAKE_SIZEOF_VOID_P ${_PYBIND11_CMAKE_SIZEOF_VOID_P})
   install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Config.cmake
                 ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake
-                tools/FindPythonLibsNew.cmake
                 tools/pybind11Tools.cmake
           DESTINATION ${PYBIND11_CMAKECONFIG_INSTALL_DIR})
 

tools/FindPythonLibsNew.cmake

@@ -13,7 +13,7 @@ if(NOT PYBIND11_PYTHON_VERSION)
 endif()
 
 set(Python_ADDITIONAL_VERSIONS 3.9 3.8 3.7 3.6 3.5 3.4)
-find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} REQUIRED)
+find_package(PythonLibs ${PYBIND11_PYTHON_VERSION} REQUIRED)
 
 include(CheckCXXCompilerFlag)
 include(CMakeParseArguments)