- Fixing utf8_general_cs according to recent changes.
- Compiling utf8_general_cs in pentium-debug-max configuration
  to avoid these problems in the future.


BUILD/compile-pentium-debug-max:
  Enable compiling of experimental collations in compile-pentium-debug-max
config/ac-macros/character_sets.m4:
  Adding hidden flag --with-experimental-collations,
  not seen in "configure --help".
strings/ctype-utf8.c:
  Compilation failure changes:
  catching up with previous character set changes:
  - uni_plane is now not a global variables
  - adding new parameter into my_strnncollsp_utf8_cs
  - adding my_strnxfrm_len into MY_COLLATION_HANDLER 
    for utf8_general_cs