On some systems, the function locale_charset() may not be exported from
libiconv but is available from libcharset, and we need -lcharset when
linking.

Introduce a make variable CHARSET_LIB that can be set to -lcharsetlib
on such systems.  Also autodetect this in the configure script by first
looking for the symbol in libiconv, and then libcharset.
Signed-off-by: NДилян Палаузов <dilyan.palauzov@aegee.org>