Fix compilation error -Werror=unused-result

Author: Lecrapouille

CMakeLists.txt

@@ -46,7 +46,7 @@ if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_CLAN
     set(THREE_SIZE_TYPE x86 CACHE INTERNAL "" FORCE)
   endif()
 
-  set(THREE_COMMON_FLAGS "${THREE_COMMON_FLAGS} -Wall -Wno-missing-braces -Wno-unused-private-field")
+  set(THREE_COMMON_FLAGS "${THREE_COMMON_FLAGS} -Wall -Wno-missing-braces -Wno-unused-private-field -Wunused-result")
 
   if(THREE_TREAT_WARNINGS_AS_ERRORS)
     set(THREE_COMMON_FLAGS "${THREE_COMMON_FLAGS} -Werror")