Fix iconv compiler warning on GCC 8

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: unrtf
 Type: Package
 Title: Extract Text from Rich Text Format (RTF) Documents
-Version: 1.3
+Version: 1.4
 Authors@R: c(
     person("Jeroen", "Ooms", ,"jeroen@berkeley.edu", role = c("aut", "cre")),
     person("Free Software Foundation, Inc", role = "cph"))
@@ -11,8 +11,9 @@ Description: Wraps the 'unrtf' utility to extract text from RTF files. Supports
 License: GPL-3
 Encoding: UTF-8
 Imports: sys (>= 2.0)
-URL: https://docs.ropensci.org/unrtf, https://github.com/ropensci/unrtf#readme (devel) 
+URL: https://docs.ropensci.org/unrtf,
+    https://github.com/ropensci/unrtf#readme (devel) 
     https://www.gnu.org/software/unrtf/ (upstream)
 BugReports: http://github.com/ropensci/unrtf/issues
-RoxygenNote: 6.0.1
+RoxygenNote: 7.0.2
 Roxygen: list(markdown = TRUE)

src/libunrtf/my_iconv.c

@@ -24,6 +24,12 @@
 
 extern int verbose_mode;
 
+#ifdef _WIN32
+#define ICONV_CONST (const char**)
+#else
+#define ICONV_CONST
+#endif
+
 /* Convert from charmap file entry to charmap table one. 
    1st byte in table entry is code length 
 */
@@ -53,7 +59,7 @@ get_code_str(FILE *f, iconv_t desc)
 		return NULL;
 	icp = utf8;
 	ocp0 = ocp = obuf + 1;
-	if (iconv(desc, &icp, &ibytes, &ocp, &obytes) == -1) {
+	if (iconv(desc, ICONV_CONST &icp, &ibytes, &ocp, &obytes) == -1) {
 		/*        fprintf(stderr, "unrtf: my_iconv: iconv error\n");*/
 		return NULL;
 	}
@@ -146,7 +152,7 @@ my_iconv(my_iconv_t cd, char **inbuf, size_t *inbytesleft, char **outbuf, size_t
 	}
 	}
 	else
-		result = iconv(cd.desc, inbuf, inbytesleft, outbuf, outbytesleft);
+		result = iconv(cd.desc, ICONV_CONST inbuf, inbytesleft, outbuf, outbytesleft);
 
 	return result;
 }