Do not assume the PRI macros are still available

[bernardladenthin]

__STDC_FORMAT_MACROS only has an effect when nothing has included
<inttypes.h> yet. NetBSD reaches the PRI macros through <sys/inttypes.h>:

#ifndef _SYS_INTTYPES_H_
#define _SYS_INTTYPES_H_
#if !defined(__cplusplus) || defined(__STDC_FORMAT_MACROS) || (__cplusplus >= 201103L)
#include <machine/int_fmtio.h>
#endif
#endif

Behind an include guard, so whichever header gets there first settles it and
the define here arrives too late. <sys/param.h> includes it, and so does
anything else that reaches <inttypes.h> — a consumer including its own
headers before utest.h is enough.

That is not hypothetical. It surfaced in sheredom/subprocess.h#113 on NetBSD
9.4 and 10.1, where test98.cpp includes subprocess.h first:

utest.h:234:22: error: expected ')' before 'PRIu64'

What this does

Requires PRId64 and PRIu64 to actually be defined before using them, and
takes the existing double fallback otherwise — a capability test rather than
a proxy for one, so include order stops mattering.

test/inttypes_include.cpp covers it the way stdint_include.c covers the
stdint case: <inttypes.h> first, then utest.h, compiled as C++98.

Verified against a stub <inttypes.h> that exposes no PRI macros, which is
what NetBSD looks like in C++98:

utest.h	result
this branch	compiles
main	error: expected ')' before 'PRIu64'

Second commit, unrelated but small

test20.cpp was compiled with -std=c++20 unconditionally. GCC 8 and clang 9
only accept -std=c++2a, and GCC 7 and older accept neither, so on those the
build stops before running anything — NetBSD 9.4 ships GCC 7.5.0. It now drops
the standard flag when neither spelling is available, which is what already
happens for test11.c without c_std_11 and on MSVC.

Happy to split that into its own PR if you would rather.

Not included

A portability matrix here, which is what found this. It needs the vendored
test/subprocess.h refreshed first — that copy predates
sheredom/subprocess.h#104, so main does not build against current glibc
without _GNU_SOURCE. Separate PR, if you want it.