Fix illegal redefinition of _Bool

[jengelh]

ldns does not build under gcc-15/glibc-2.43. The error is:

.//libtool --tag=CC --quiet --mode=compile gcc -I. -I.  -DHAVE_CONFIG_H -DLDNS_TRUST_ANCHOR_FILE="\"/etc/unbound/root.key\"" -DOPENSSL_API_COMPAT=10100 -fno-strict-aliasing -Wunused-function -Wstrict-prototypes -Wwrite-strings -W -Wall -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g -fno-strict-aliasing -D_GNU_SOURCE -I./include/ldns  -I/usr/include/python3.13 -Wno-unused-variable -Wno-unused-parameter -Wno-missing-field-initializers -fno-strict-aliasing -c ./contrib/python/ldns_wrapper.c -o ldns_wrapper.lo
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:114:14: error: conflicting types for '__assert_single_arg'; have 'signed char(signed char)'
  114 | extern _Bool __assert_single_arg (_Bool);
      |              ^~~~~~~~~~~~~~~~~~~
In file included from /usr/include/python3.13/Python.h:20,
		 from ./contrib/python/ldns_wrapper.c:203:
/usr/include/assert.h:114:14: note: previous declaration of '__assert_single_arg' with type '_Bool(_Bool)'
  114 | extern _Bool __assert_single_arg (_Bool);
      |              ^~~~~~~~~~~~~~~~~~~

The reduced test case is:

#include <assert.h>
#define _Bool signed char
#include <assert.h>
// gcc-15 -c 1.c

Identifiers starting with an underscore are reserved for the implementation. Redefinining them can break compilation in funny ways (like this).

Stop redefinining/redeclaring internal identifiers. Delete references to the autoconf manual, because they too have removed that ugly snippet in commit v2.72a-68-g6f2aadd7.