c++: beware of -Wshadow

This line:

    slice<stack_symbol_type, stack_type> slice (yystack_, yylen);

triggers warnings:

    parse.h:1790:11: note: shadowed declaration is here

Reported by Frank Heckenbach.
http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00002.html

* configure.ac (warn_c): Move -Wshadow to...
(warn_common): here.
* data/skeletons/stack.hh (slice): Define as an inner class of stack.
* data/skeletons/lalr1.cc: Adjust.
Rename the variable as 'range' instead of 'slice'.

configure.ac

@@ -96,8 +96,8 @@ AM_CONDITIONAL([ENABLE_GCC_WARNINGS], [test "$enable_gcc_warnings" = yes])
 if test "$enable_gcc_warnings" = yes; then
   warn_common='-Wall -Wextra -Wno-sign-compare -Wcast-align
     -fparse-all-comments -Wdocumentation
-    -Wformat -Wnull-dereference -Wpointer-arith -Wwrite-strings'
-  warn_c='-Wbad-function-cast -Wshadow -Wstrict-prototypes'
+    -Wformat -Wnull-dereference -Wpointer-arith -Wshadow -Wwrite-strings'
+  warn_c='-Wbad-function-cast -Wstrict-prototypes'
   warn_cxx='-Wnoexcept'
   # Warnings for the test suite only.
   #