[tlbuild] -funsigned-char is not portable

[Vladimir Volovich]

Hi!

> the proper flag for this compiler is -qchars=unsigned which should be
> tested via configure (if there is such a test), or -funsigned-char
> should be removed.

there is a test which is close to this one (but not quite the same, as
it also suppresses warnings about "usage of char-type to index a value-field"):

http://www.nongnu.org/autoconf-archive/ac_prog_cc_char_subscripts.html

Best,
v.