Changed the string_addins.h to check for the FreeBSD version in order
to check the version when `strlcat()' and `strlcpy()' were introduced.
Also added macros to check if the operating system is Mac OS.

Also modified the header to check for operating systems only when the
macro `HAS_LESS_ERROR_PRONE_STR_FUNCS' is not defined, with this users
can check for the availability of this functions using a tool such as
CMake.