Build: suppress warnings from 3rd party libraries

Warnings from 3rd parties are not relevant and are only polluting the warning output. Marking these libraries as SYSTEM suppresses warning.

https://gcc.gnu.org/onlinedocs/cpp/Invocation.html#index-I
https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html

src/Mod/Spreadsheet/App/CMakeLists.txt

@@ -5,6 +5,10 @@ endif(WIN32)
 include_directories(
     ${CMAKE_CURRENT_SOURCE_DIR}
     ${CMAKE_CURRENT_BINARY_DIR}
+)
+
+include_directories(
+    SYSTEM
     ${PYCXX_INCLUDE_DIR}
     ${Python3_INCLUDE_DIRS}
     ${Boost_INCLUDE_DIRS}

src/Mod/Spreadsheet/Gui/CMakeLists.txt

@@ -1,6 +1,10 @@
 include_directories(
     ${CMAKE_CURRENT_BINARY_DIR}
     ${CMAKE_CURRENT_SOURCE_DIR}
+)
+
+include_directories(
+    SYSTEM
     ${Boost_INCLUDE_DIRS}
     ${COIN3D_INCLUDE_DIRS}
     ${ZIPIOS_INCLUDES}