CMake: make unsequenced modifications an error

They not only result in discrepancies between different compiler,
but trigger undefined behavior. Avoid them by having them break
the CMake build and CI.

CMakeLists.txt

@@ -19,6 +19,7 @@ endif()
 include(AddIfFlagCompiles)
 add_if_flag_compiles(-Werror=pointer-arith CMAKE_C_FLAGS)
 add_if_flag_compiles(-Werror=implicit-function-declaration CMAKE_C_FLAGS)
+add_if_flag_compiles(-Werror=unsequenced CMAKE_C_FLAGS)
 # src/external/jar_xm.h does shady stuff
 add_if_flag_compiles(-fno-strict-aliasing CMAKE_C_FLAGS)