build failure when system does not provide MB_CUR_MAX

[Mike Frysinger]

looking through the gnulib source code, i dont see any checks/replacements for 
when MB_CUR_MAX does not exist (like in a uClibc build where all multibyte 
related stuff has been disabled).  for example, zile falls apart like so:
../../zile-2.3.7/lib/mbrtowc.c: In function ‘rpl_mbrtowc’:
../../zile-2.3.7/lib/mbrtowc.c:96: warning: implicit declaration of function 
‘mbtowc’
../../zile-2.3.7/lib/mbrtowc.c:124: error: ‘MB_CUR_MAX’ undeclared (first use 
in this function)
../../zile-2.3.7/lib/mbrtowc.c:124: error: (Each undeclared identifier is 
reported only once
../../zile-2.3.7/lib/mbrtowc.c:124: error: for each function it appears in.) 
make[6]: *** [mbrtowc.o] Error 1

shouldnt the multibyte replacements in gnulib be testing for MB_CUR_MAX and 
provide a replacement when it doesnt exist ?  after all, gnulib is providing 
the multibyte functions itself, so it should know full well the value of 
MB_CUR_MAX ...
-mike