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/MeshPart/App/CMakeLists.txt

@@ -12,6 +12,10 @@ endif(BUILD_FEM_NETGEN)
 include_directories(
     ${CMAKE_BINARY_DIR}
     ${CMAKE_SOURCE_DIR}/src
+)
+
+include_directories(
+    SYSTEM
     ${Boost_INCLUDE_DIRS}
     ${OCC_INCLUDE_DIR}
     ${ZLIB_INCLUDE_DIR}
@@ -135,7 +139,7 @@ if (BUILD_FLAT_MESH)
             endif()
 
             if(Boost_FOUND)
-                include_directories(${Boost_INCLUDE_DIRS})
+                include_directories(SYSTEM ${Boost_INCLUDE_DIRS})
                 SET(FLATMESH_SRCS
                     MeshFlattening.cpp
                     MeshFlattening.h

src/Mod/MeshPart/Gui/CMakeLists.txt

@@ -13,6 +13,10 @@ include_directories(
     ${CMAKE_BINARY_DIR}
     ${CMAKE_SOURCE_DIR}/src
     ${CMAKE_CURRENT_BINARY_DIR}
+)
+
+include_directories(
+    SYSTEM
     ${Boost_INCLUDE_DIRS}
     ${OCC_INCLUDE_DIR}
     ${COIN3D_INCLUDE_DIRS}