GCC15(-std=gnu23) adjustment: Fix function pointer cast in qsort()

[charmitro]

1. Suppress -Wold-style-definition warnings globally since we prefer old-style definitions.
2. Fix hlink.c's cast in qsort() call to properly declare type.
3. Fix pool_alloc.c's function pointer type to match its usage.
4. Fix syscall.c's lseek64 forward declarations.
5. Fix bool typedef conflict in wildtest.c by renaming to rsync_bool.

This cleans up the build with modern compilers (GCC15) while maintaining compatibility with older versions. The qsort comparison function cast in particular is now more type-safe.

Closes #664

[charmitro]

PR updated to only fix a pointer cast in qsort() in hlink.c:

rsync/hlink.c

Line 120 in 321dd78

qsort(ndx_list, ndx_count, sizeof ndx_list[0], (int (*)()) hlink_compare_gnum);

hlink.c: In function ‘match_gnums’:
hlink.c:120:56: warning: cast between incompatible function types from ‘int (*)(int *, int *)’ to ‘int (*)(void)’ [-Wcast-function-type]
  120 |         qsort(ndx_list, ndx_count, sizeof ndx_list[0], (int (*)()) hlink_compare_gnum);
      |                                                        ^
hlink.c:120:56: error: passing argument 4 of ‘qsort’ from incompatible pointer type [-Wincompatible-pointer-types]
  120 |         qsort(ndx_list, ndx_count, sizeof ndx_list[0], (int (*)()) hlink_compare_gnum);
      |                                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                        |
      |                                                        int (*)(void)
In file included from rsync.h:328,
                 from hlink.c:23:
/usr/include/stdlib.h:971:34: note: expected ‘__compar_fn_t’ {aka ‘int (*)(const void *, const void *)’} but argument is of type ‘int (*)(void)’
  971 |                    __compar_fn_t __compar) __nonnull ((1, 4));
      |                    ~~~~~~~~~~~~~~^~~~~~~~