Utilize stdbool.h unconditionally

[floppym]

This resolves a build failure when using a C23 compiler and pyldns is enabled.

In file included from ./ldns/buffer.h:16,
                 from ./include/ldns/ldns.h:96,
                 from ../contrib/python/ldns_wrapper.c:3547:
/usr/include/assert.h:115:14: error: conflicting types for '__assert_single_arg'; have 'signed char(signed char)'
  115 | extern _Bool __assert_single_arg (_Bool);
      |              ^~~~~~~~~~~~~~~~~~~
In file included from /usr/include/python3.14/Python.h:20,
                 from ../contrib/python/ldns_wrapper.c:203:
/usr/include/assert.h:115:14: note: previous declaration of '__assert_single_arg' with type 'bool(bool)'
  115 | extern _Bool __assert_single_arg (_Bool);
      |              ^~~~~~~~~~~~~~~~~~~
make: *** [Makefile:329: ldns_wrapper.lo] Error 1

Also, the autoconf-2.73 documentation suggests that any reasonably modern C compiler will supply stdbool.h. The AC_HEADER_STDBOOL macro and the HAVE_STDBOOL_H macro are obsolete.

The HAVE_STDBOOL_H macro was unusable in installed headers anyway, so this makes the headers behave more consistently all around.