Install pybind header in subdirectory of python include directories

Author: SylvainCorlay

CMakeLists.txt

@@ -110,7 +110,7 @@ endif()
 
 if (PYBIND11_INSTALL)
   install(FILES ${PYBIND11_HEADERS}
-          DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/pybind11)
+      DESTINATION ${PYTHON_INCLUDE_DIRS}/pybind11)
   # GNUInstallDirs "DATADIR" wrong here; CMake search path wants "share".
   set(PYBIND11_CMAKECONFIG_INSTALL_DIR "share/cmake/${PROJECT_NAME}" CACHE STRING "install path for pybind11Config.cmake")