dosfstools: switch to AC_CHECK_LIB

This fixes spurious build-errors on OpenWrt, where the AM_ICONV macro
is undefined while invoking autoconfig. Later in the build, the ICONV
LDOPTIONS are set to @LIBICONV@, failing the build.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 9300a20dcc)

tools/dosfstools/patches/101-config-switch-to-AC_CHECK_LIB.patch

@@ -0,0 +1,28 @@
+From e7671c2a3be03d790cbc225cd3e784b5434fb5da Mon Sep 17 00:00:00 2001
+From: David Bauer <mail@david-bauer.net>
+Date: Mon, 16 Jan 2023 01:29:22 +0100
+Subject: [PATCH] config: switch to AC_CHECK_LIB
+
+This fixes spurious build-errors on OpenWrt, where the AM_ICONV macro
+is undefined while invoking autoconfig. Later in the build, the ICONV
+LDOPTIONS are set to @LIBICONV@, failing the build.
+
+Signed-off-by: David Bauer <mail@david-bauer.net>
+---
+ configure.ac | 5 +----
+ 1 file changed, 1 insertion(+), 4 deletions(-)
+
+--- a/configure.ac
++++ b/configure.ac
+@@ -70,10 +70,7 @@ AC_CHECK_DECLS([getmntent], [], [], [[#i
+ AC_CHECK_DECLS([getmntinfo], [], [], [[#include <sys/mount.h>]])
+ 
+ # optional iconv support
+-AC_ARG_WITH([iconv], AS_HELP_STRING([--without-iconv], [build without iconv support]))
+-if test "x$with_iconv" != "xno"; then
+-	AM_ICONV
+-fi
++AC_CHECK_LIB(iconv, iconv_open)
+ 
+ # xxd (distributed with vim) is used in the testsuite
+ AC_CHECK_PROG([XXD_FOUND], [xxd], [yes])