Evidence that __STDC__ should be used properly

[Doug Gwyn]

Here is additional evidence why it is NOT "helpful" for C compilers that
provide support for functions prototypes to advertise themselves as "ANSI
C compatible" via __STDC__ when they don't fully conform to the Standard:

	[Extracted from a trip report I recently received:]

	The equipment available for the feasibility study was an Apollo
	DN-1000, running the very latest version of UNIX from Apollo,
	version SR10.1.  ... our first task was to "port" the software
	over.  This took a significant amount of time, owing to various
	deficiencies in the Apollo software.  ...

	The C compiler on the Apollo identifies itself as an ANSI-Standard
	compiler (__STDC__ = 1), yet numerous ANSI features were missing,
	including "#" token-pasting and the "const" keyword.  In addition,
	the ANSI header files stdlib.h, stdarg.h, stddef.h did not exist.
	The ANSI "stdarg" implementation (using the "..." notation) did
	not work.  The older "varargs" implementation conflicted with all
	the function prototypes.  With -U__STDC__, the old "varargs"
	package came to life.