Restore build on QNX Neutrino 6.5.0 (re: a874f8c1)

The QNX system at polarhome.com seems to be back up, at least
temporarily, though polarhome has officially shut down. This
allowed me to test ksh on QNX again, discovering that the build
was broken since reworking the standards macros handling.

src/lib/libast/features/standards:
- On QNX, define _QNX_SOURCE=1 to enable all extensions and
  _FILE_OFFSET_BITS=64 to enable large file support with standard
  library function names.

src/cmd/ksh93/README

@@ -279,7 +279,7 @@ failures (crashes, and/or important functionality does not work).
 *	NetBSD 9.2 on x86_64
 *	OpenBSD 6.8 on x86_64
 	OpenBSD 7.0 on x86_64
-**	QNX 6.5.0 on i386
+*	QNX Neutrino 6.5.0 on i386
 *	Solaris 11.4 (gcc) on x86_64
 	Solaris 11.4 (Solaris Studio 12.5 cc) on x86_64
 *	UnixWare 7.1.4 on x86

src/lib/libast/features/standards

@@ -165,6 +165,22 @@ elif tst note{ GNU (glibc) or Cygwin }end compile{
 		#endif
 		#define basename	basename	/* avoid string.h defining this in conflict with AST basename(3) */
 	}
+elif tst note{ QNX }end compile{
+		/*
+		 * QNX Neutrino, tested on version 6.5.0 as of April 2022.
+		 */
+		#define _QNX_SOURCE	1
+		#if !__QNX__
+		#error not QNX
+		#endif
+	}end {
+		#ifndef _QNX_SOURCE
+		#define _QNX_SOURCE	1
+		#endif
+		#ifndef _FILE_OFFSET_BITS
+		#define _FILE_OFFSET_BITS 64
+		#endif
+	}
 elif tst note{ _ALL_SOURCE & _POSIX_SOURCE & _POSIX_C_SOURCE & _XOPEN_SOURCE & __EXTENSIONS__ works }end compile{
 		#define _ALL_SOURCE	1
 		#define _POSIX_SOURCE	1