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/Tools/embedded/Qt/cxx/CMakeLists.txt

@@ -30,6 +30,11 @@ set(FREECAD_LIBPACK_DIR ${CMAKE_SOURCE_DIR} CACHE PATH  "Directory of the FreeCA
 add_definitions(-DBOOST_ALL_DYN_LINK)
 
 include_directories(
+    ${FREECAD_SRC_DIR}
+)
+
+include_directories(
+    SYSTEM
     ${Boost_INCLUDE_DIRS}
     #${COIN3D_INCLUDE_DIRS}
     ${PYCXX_INCLUDE_DIR}
@@ -37,7 +42,6 @@ include_directories(
     ${Qt5Core_INCLUDE_DIRS}
     ${Qt5Gui_INCLUDE_DIRS}
     ${Qt5Widgets_INCLUDE_DIRS}
-    ${FREECAD_SRC_DIR}
     "${FREECAD_LIBPACK_DIR}/include"
 )