FTBFS with glibc 2.43 due to ISO C23 const return types

[bkuhls]

The package can not be compiled with glibc 2.43:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1128750

| acl.c: In function ‘parse_addr_cidr’:
| acl.c:62:9: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
|    62 |     pch = strchr(str, '/');
|       |         ^
| netutils.c: In function ‘validate_hostname’:
| netutils.c:278:28: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
|   278 |         char *next_dot   = strchr(label, '.');
|       |                            ^~~~~~
| netutils.c: In function ‘validate_hostname’:
| netutils.c:278:28: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
|   278 |         char *next_dot   = strchr(label, '.');
|       |                            ^~~~~~

...
| netutils.c: In function ‘validate_hostname’:
| netutils.c:278:28: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
|   278 |         char *next_dot   = strchr(label, '.');
|       |                            ^~~~~~

...

The issue is due to ISO C23 declaration of bsearch, memchr, strchr,
strpbrk, strrchr, strstr, wcschr, wcspbrk, wcsrchr, wcsstr and wmemchr,
which now returns a pointer to a const-qualified type when the input
argument is a pointer to a const-qualified type: https://sourceware.org/git/?p=glibc.git;a=blob;f=NEWS;h=e271fb2e4d76903c77a302aaec1ca22ce31027d0;hb=f762ccf84f122d1354f103a151cba8bde797d521#l19