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

tests/src/Mod/MeshPart/CMakeLists.txt

@@ -1,5 +1,5 @@
 
-target_include_directories(MeshPart_tests_run PUBLIC
+target_include_directories(MeshPart_tests_run SYSTEM PUBLIC
     ${EIGEN3_INCLUDE_DIR}
     ${OCC_INCLUDE_DIR}
     ${PYCXX_INCLUDE_DIR}