maint: use "const" and "pure" function attributes where possible

* configure.ac (WARN_CFLAGS): Add -Wsuggest-attribute=const,
-Wsuggest-attribute=pure and -Wsuggest-attribute=noreturn.
(GNULIB_WARN_CFLAGS): But do not add them here... yet.
* src/chown-core.h (chopt_free, uid_to_name): Add function attribute(s).
* src/copy.c (is_ancestor, valid_options): Likewise.
* src/copy.h (chown_failure_ok): Likewise.
* src/dd.c (operand_matches, operand_is): Likewise.
* src/df.c (selected_fstype, excluded_fstype): Likewise.
* src/expr.c (null looks_like_integer): Likewise.
* src/md5sum.c (hex_digits): Likewise.
* src/od.c (get_lcm): Likewise.
* src/pathchk.c (component_start, component_len): Likewise.
* src/pinky.c (count_ampersands): Likewise.
* src/pr.c (cols_ready_to_print): Likewise.
* src/ptx.c (search_table): Likewise.
* src/sort.c (find_unit_order): Likewise.
* src/stty.c (mode_type_flag, string_to_baud, baud_to_value): Likewise.
* src/system.h (gcd, lcm): Likewise.
* src/tr.c (is_char_class_member, look_up_char_class): Likewise.
(star_digits_closebracket): Likewise.
* src/uniq.c (find_field): Likewise.
* src/wc.c (compute_number_width): Likewise.
* lib/xfts.h (cycle_warning_required): Likewise.
* gl/lib/randint.h (randint_get_source): Likewise.
* gl/lib/randperm.c (ceil_lg): Likewise.
* gl/lib/randperm.h (randperm_bound): Likewise.
* lib/strnumcmp.h (strintcmp): Likewise.

gl/lib/randint.h

@@ -34,7 +34,8 @@ struct randint_source;
 
 struct randint_source *randint_new (struct randread_source *);
 struct randint_source *randint_all_new (char const *, size_t);
-struct randread_source *randint_get_source (struct randint_source const *);
+struct randread_source *randint_get_source (struct randint_source const *)
+  _GL_ATTRIBUTE_PURE;
 randint randint_genmax (struct randint_source *, randint genmax);
 
 /* Consume random data from *S to generate a random number in the range

gl/lib/randperm.c

@@ -30,7 +30,7 @@
 /* Return the ceiling of the log base 2 of N.  If N is zero, return
    an unspecified value.  */
 
-static size_t
+static size_t _GL_ATTRIBUTE_CONST
 ceil_lg (size_t n)
 {
   size_t b = 0;

gl/lib/randperm.h

@@ -1,4 +1,4 @@
 #include "randint.h"
 #include <stddef.h>
-size_t randperm_bound (size_t, size_t);
+size_t randperm_bound (size_t, size_t) _GL_ATTRIBUTE_CONST;
 size_t *randperm_new (struct randint_source *, size_t, size_t);

gl/lib/tempname.c.diff

@@ -22,7 +22,7 @@ index 2da5afe..562955a 100644
  }
  #endif /* _LIBC */
 
-+static inline bool
++static inline bool _GL_ATTRIBUTE_PURE
 +check_x_suffix (char const *s, size_t len)
 +{
 +  return len <= strspn (s, "X");