Core: Move PyCXX library to src/3rdParty.

src/Mod/Start/App/CMakeLists.txt

@@ -22,6 +22,7 @@
 #  ***************************************************************************/
 
 include_directories(
+        ${PYCXX_INCLUDE_DIR}
         ${Python3_INCLUDE_DIRS}
         ${QtCore_INCLUDE_DIRS}
         ${ZIPIOS_INCLUDES}

src/Mod/Start/Gui/CMakeLists.txt

@@ -25,6 +25,7 @@ include_directories(
         ${CMAKE_CURRENT_BINARY_DIR}
         ${Boost_INCLUDE_DIRS}
         ${COIN3D_INCLUDE_DIRS}
+        ${PYCXX_INCLUDE_DIR}
         ${Python3_INCLUDE_DIRS}
         ${QtCore_INCLUDE_DIRS}
         ${QtSvg_INCLUDE_DIRS}