The standard iconv function uses char ** even for the input
buffer. Using the incompatible const char ** type causes the check to
fail if the compiler treats such type errors as errors, instead of
merely warning about it.

Submitted upstream: <https://github.com/proftpd/proftpd/pull/1754>

diff --git a/configure b/configure
index 433bb928..1a0538d8 100755
--- a/configure
+++ b/configure
@@ -21475,7 +21475,7 @@ main ()
 {
 
     size_t res, in_len = 0, out_len = 0;
-    const char *in = NULL;
+    char *in = NULL;
     char *out = NULL;
     res = iconv((iconv_t)-1, &in, &in_len, &out, &out_len);
 
diff --git a/configure.in b/configure.in
index e2afbfa2..8a6ef35c 100644
--- a/configure.in
+++ b/configure.in
@@ -2100,7 +2100,7 @@ AC_TRY_LINK(
   ],
   [ 
     size_t res, in_len = 0, out_len = 0;
-    const char *in = NULL;
+    char *in = NULL;
     char *out = NULL;
     res = iconv((iconv_t)-1, &in, &in_len, &out, &out_len);
   ],