autoconf: Checks for some library functions.

autoconf: Checks for some library functions.

./configure script checks now for the following library functions:
 * strcasestr (NO_STRCASESTR)
 * strlcpy (NO_STRLCPY)
 * setenv (NO_SETENV)
in default C library and in libraries which have AC_CHECK_LIB done for
them.

Checks not implemented:
 * NO_MMAP  - probably only via optional features configuration
 * NO_IPV6  - what does "lack IPv6 support" mean?
 * NO_ICONV - what does "properly support iconv" mean?

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>