Revert "CMake: make unsequenced modifications an error"

This reverts commit 23c1c0bdb2.

Because add_if_flag_compiles apparently allows unsequenced
even if the compiler doesn't support it..

CMakeLists.txt

@@ -19,7 +19,6 @@ 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)