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/Main/CMakeLists.txt

@@ -5,6 +5,7 @@ file(COPY icon.ico DESTINATION ${CMAKE_CURRENT_BINARY_DIR})
 
 if(BUILD_GUI)
     include_directories(
+        SYSTEM
         ${Boost_INCLUDE_DIRS}
         ${COIN3D_INCLUDE_DIRS}
         ${ZLIB_INCLUDE_DIR}
@@ -16,6 +17,7 @@ if(BUILD_GUI)
     )
 else(BUILD_GUI)
     include_directories(
+        SYSTEM
         ${Boost_INCLUDE_DIRS}
         ${ZLIB_INCLUDE_DIR}
         ${QT_INCLUDE_DIR}