build: check -Wmissing-prototypes

pstate_clear is lacking a prototype.
Reported by Ryan
https://lists.gnu.org/r/bug-bison/2020-05/msg00101.html

Besides, none of the C examples were compiled with the warning flags.

* configure.ac (warn_c): Add -Wmissing-prototypes.
* data/skeletons/yacc.c (pstate_clear): Make it static.
* examples/local.mk (TEST_CFLAGS): New.
* examples/c/bistromathic/local.mk, examples/c/calc/local.mk,
* examples/c/lexcalc/local.mk, examples/c/mfcalc/local.mk,
* examples/c/pushcalc/local.mk, examples/c/reccalc/local.mk,
* examples/c/rpcalc/local.mk:
Use it.

GCC's warn_unused_result is not silenced by a cast to void, so we have
to "use" scanf's result.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

Flex generated code produces too many warnings, including things such
as, with ICC:

    examples/c/lexcalc/scan.c(1088): error #1682: implicit conversion
              of a 64-bit integral type to a smaller integral type (potential portability problem)
    2259                YY_INPUT( (&YY_CURRENT_BUFFER_LVALUE->yy_ch_buf[number_to_move]),
    2260                ^
    2261
    2262

I am tired of trying to fix Flex's output.  The project does not seem
maintained.  We ought to avoid it.  So, for the time being, don't try
to enable warnings with Flex.

* examples/c/bistromathic/parse.y, examples/c/reccalc/scan.l: Fix
warnings.
* doc/bison.texi: Discard scanf's return value to defeat
-Werror=unused-result.

doc/bison.texi

@@ -1879,6 +1879,7 @@ Here is the code for the lexical analyzer:
    and tabs, and returns 0 for end-of-input. */
 
 #include <ctype.h>
+#include <stdlib.h>
 @end group
 
 @group
@@ -1895,7 +1896,8 @@ yylex (void)
   if (c == '.' || isdigit (c))
     @{
       ungetc (c, stdin);
-      scanf ("%lf", &yylval);
+      if (scanf ("%lf", &yylval) != 1)
+        abort ();
       return NUM;
     @}
 @end group
@@ -2728,7 +2730,8 @@ yylex (void)
   if (c == '.' || isdigit (c))
     @{
       ungetc (c, stdin);
-      scanf ("%lf", &yylval.NUM);
+      if (scanf ("%lf", &yylval.NUM) != 1)
+        abort ();
       return NUM;
     @}
 @end group
@@ -2755,10 +2758,10 @@ Bison generated a definition of @code{YYSTYPE} with a member named
           if (bufsize <= i)
             @{
               bufsize = 2 * bufsize + 40;
-              symbuf = realloc (symbuf, bufsize);
+              symbuf = realloc (symbuf, (size_t) bufsize);
             @}
           /* Add this character to the buffer. */
-          symbuf[i++] = c;
+          symbuf[i++] = (char) c;
           /* Get another character. */
           c = getchar ();
         @}
@@ -10629,7 +10632,7 @@ when there were errors.  No file was generated (except the reports generated
 by @option{--verbose}, etc.).  In particular, the output files that possibly
 existed were not changed.
 
-@item 63 (mistmatch)
+@item 63 (mismatch)
 when @command{bison} does not meet the version requirements of the grammar
 file. @xref{Require Decl}.  No file was generated or changed.
 @end table
@@ -15269,6 +15272,7 @@ London, Department of Computer Science, TR-00-12 (December 2000).
 @c LocalWords: YYUNDEF SymbolKind yypcontext YYENOMEM TOKENMAX getBundle
 @c LocalWords: ResourceBundle myResources getString getName getToken
 @c LocalWords: getLocation getExpectedTokens reportSyntaxError bistromathic
+@c LocalWords: TokenKind
 
 @c Local Variables:
 @c ispell-dictionary: "american"