Fix misnamed variable

The same variable was used to store two different sets of compiler
flags.

CMakeLists.txt

@@ -399,8 +399,8 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   endif(NOT ${CMAKE_BUILD_TYPE} STREQUAL "Debug")
 
   if(FORCE_32BIT)
-    ADD_C_COMPILER_FLAG_IF_AVAILABLE("-m32" HAVE_WNO_UNUSED_BUT_SET_VARIABLE)
+    ADD_C_COMPILER_FLAG_IF_AVAILABLE("-m32" HAVE_M32)
-    ADD_CXX_COMPILER_FLAG_IF_AVAILABLE("-m32" HAVE_WNO_UNUSED_BUT_SET_VARIABLE)
+    ADD_CXX_COMPILER_FLAG_IF_AVAILABLE("-m32" HAVE_M32)
     set(CMAKE_SHARED_LINKER_FLAGS "-m32")
   endif(FORCE_32BIT)