headers: move CPP guards into YY_*_INCLUDED to avoid collisions

See <http://lists.gnu.org/archive/html/bug-bison/2012-09/msg00016.html>.

* data/c.m4 (b4_cpp_guard): Prepend YY_ and append _INCLUDED.
* tests/headers.at: Adjust.
* NEWS, doc/bison.texi: Document.

doc/bison.texi

@@ -5124,6 +5124,23 @@ Values, ,Semantic Values of Tokens}.
 If you have declared @code{%code requires} or @code{%code provides}, the output
 header also contains their code.
 @xref{%code Summary}.
+
+@cindex Header guard
+The generated header is protected against multiple inclusions with a C
+preprocessor guard: @samp{YY_@var{PREFIX}_@var{FILE}_INCLUDED}, where
+@var{PREFIX} and @var{FILE} are the prefix (@pxref{Multiple Parsers,
+,Multiple Parsers in the Same Program}) and generated file name turned
+uppercase, with each series of non alphanumerical characters converted to a
+single underscore.
+
+For instance with @samp{%define api.prefix "calc"} and @samp{%defines
+"lib/parse.h"}, the header will be guarded as follows.
+@example
+#ifndef YY_CALC_LIB_PARSE_H_INCLUDED
+# define YY_CALC_LIB_PARSE_H_INCLUDED
+...
+#endif /* ! YY_CALC_LIB_PARSE_H_INCLUDED */
+@end example
 @end deffn
 
 @deffn {Directive} %defines @var{defines-file}