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.

Signed-off-by: Дилян Палаузов <dilyan.palauzov@aegee.org>
diff --git a/config.mak.in b/config.mak.in
index 10698c8..b2ba710 100644
--- a/config.mak.in
+++ b/config.mak.in
@@ -74,3 +74,4 @@
 NO_PTHREADS=@NO_PTHREADS@
 PTHREAD_CFLAGS=@PTHREAD_CFLAGS@
 PTHREAD_LIBS=@PTHREAD_LIBS@
+CHARSET_LIB=@CHARSET_LIB@