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.

1 files changed, 1 insertions, 0 deletions

diff --git a/examples/c/calc/local.mk b/examples/c/calc/local.mk
index 14b78f3b..503b034a 100644
--- a/examples/c/calc/local.mk
+++ b/examples/c/calc/local.mk
@@ -27,6 +27,7 @@ nodist_%C%_calc_SOURCES = %D%/calc.y
 
 # Don't use gnulib's system headers.
 %C%_calc_CPPFLAGS = -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
+%C%_calc_CFLAGS = $(TEST_CFLAGS)
 
 dist_calc_DATA = %D%/calc.y %D%/Makefile %D%/README.md
 CLEANFILES += %D%/calc.[ch] %D%/calc.output %D%/scan.c