Enable sbrk() as an option.

DefineOptions.cmake

@@ -86,6 +86,8 @@ endif(BLOCKS_ORGANIZATION STREQUAL "dll")
 
 if(WITH_SYSTEM_CALLS STREQUAL "mmap")
   set(WITH_MMAP ON)
+elseif(WITH_SYSTEM_CALLS STREQUAL "sbrk")
+  set(WITH_MMAP OFF)
 endif(WITH_SYSTEM_CALLS STREQUAL "mmap")
 
 if(SEARCH_POLICY STREQUAL "best")