what is $ac_t ?

[Sam Steingold]

google appears to suggest that $ac_t is widely used in older configure scripts 
as a tab character, e.g.:
echo "$ac_t""found" 1>&6
it appears that it it not mentioned in any docs and not defined to anything 
useful for some time.

since recently, however, it is used in a loop setting ac_install_sh, and now it 
is set to "install-sh". thus now I see configure messages like

checking for getrlimit declaration... (cached) install-shextern int getrlimit 
(enum __rlimit_resource, struct rlimit *);

Thus my questions are:

1. optional: what is the history of the matter?

2. required: is there a replacement for ac_t or should it be dropped?

thanks!
Sam.