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>

configure.ac

@@ -836,6 +836,16 @@ AC_CHECK_HEADER([libcharset.h],
 [HAVE_LIBCHARSET_H=YesPlease],
 [HAVE_LIBCHARSET_H=])
 AC_SUBST(HAVE_LIBCHARSET_H)
+# Define CHARSET_LIB if libiconv does not export the locale_charset symbol
+# and libcharset does
+CHARSET_LIB=
+AC_CHECK_LIB([iconv], [locale_charset],
+       [],
+       [AC_CHECK_LIB([charset], [locale_charset],
+                     [CHARSET_LIB=-lcharset])
+       ]
+)
+AC_SUBST(CHARSET_LIB)
 #
 # Define NO_STRCASESTR if you don't have strcasestr.
 GIT_CHECK_FUNC(strcasestr,