Makefile: introduce CHARSET_LIB to link with -lcharset
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.