mandoc: Const correctness for C23

On some platforms, e.g. Linux Clang 22.1.8 / glibc 2.43, strchr()
now implements the C23 behaviour where passing a const pointer to
strchr() also returns a const pointer.  This breaks mandoc during
the bootstrap build, since it assumes the return value is always
a mutable pointer.

In read.c, make the existing temporary pointer const, and for the
mandoc_asprintf() call, add a new mutable local.

In mdoc.c and out.c, since the data is mutable and is mutated here,
remove const from the temporary pointers.

MFC after:	1 week
Reviewed by:	fuz
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D58495

3 files changed, 9 insertions, 8 deletions

diff --git a/contrib/mandoc/mdoc.c b/contrib/mandoc/mdoc.c
index 475a6aa0ddb3..90a079870f64 100644
--- a/contrib/mandoc/mdoc.c
+++ b/contrib/mandoc/mdoc.c
@@ -165,8 +165,7 @@ static int
 mdoc_ptext(struct roff_man *mdoc, int line, char *buf, int offs)
 {
 	struct roff_node *n;
-	const char	 *cp, *sp;
-	char		 *c, *ws, *end;
+	char		 *c, *ws, *end, *cp, *sp;
 
 	n = mdoc->last;
 
diff --git a/contrib/mandoc/out.c b/contrib/mandoc/out.c
index 21c282b2141b..082e7e61e8af 100644
--- a/contrib/mandoc/out.c
+++ b/contrib/mandoc/out.c
@@ -457,8 +457,8 @@ static size_t
 tblcalc_literal(struct rofftbl *tbl, struct roffcol *col,
     const struct tbl_dat *dp, size_t mw)
 {
-	const char	*str;	/* Beginning of the first line. */
-	const char	*beg;	/* Beginning of the current line. */
+	char		*str;	/* Beginning of the first line. */
+	char		*beg;	/* Beginning of the current line. */
 	char		*end;	/* End of the current line. */
 
 	/* Widths in basic units. */
diff --git a/contrib/mandoc/read.c b/contrib/mandoc/read.c
index 0e8d32df5a5e..d958a25832b3 100644
--- a/contrib/mandoc/read.c
+++ b/contrib/mandoc/read.c
@@ -628,7 +628,7 @@ mparse_readfd(struct mparse *curp, int fd, const char *filename)
 int
 mparse_open(struct mparse *curp, const char *file)
 {
-	char		 *cp;
+	const char	 *cp;
 	int		  fd, save_errno;
 
 	cp = strrchr(file, '.');
@@ -645,10 +645,12 @@ mparse_open(struct mparse *curp, const char *file)
 	 */
 
 	if ( ! curp->gzip) {
+		char	*fn;
+
 		save_errno = errno;
-		mandoc_asprintf(&cp, "%s.gz", file);
-		fd = open(cp, O_RDONLY);
-		free(cp);
+		mandoc_asprintf(&fn, "%s.gz", file);
+		fd = open(fn, O_RDONLY);
+		free(fn);
 		errno = save_errno;
 		if (fd != -1) {
 			curp->gzip = 1;