c++: avoid -Wundefined-func-template warnings from clang

Reported by Derek Clegg.
http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00006.html

Clang does not like this:

    template <typename D>
    struct basic_symbol : D
    {
      basic_symbol();
    };

    struct by_type {};

    struct symbol_type : basic_symbol<by_type>
    {
      symbol_type(){}
    };

It gives:

    $ clang++-mp-7.0 -Wundefined-func-template foo.cc -c
    foo.cc:11:3: warning: instantiation of function 'basic_symbol<by_type>::basic_symbol'
                 required here, but no definition is available [-Wundefined-func-template]
      symbol_type(){}
      ^
    foo.cc:4:3: note: forward declaration of template entity is here
      basic_symbol();
      ^
    foo.cc:11:3: note: add an explicit instantiation declaration to suppress this warning
                 if 'basic_symbol<by_type>::basic_symbol' is explicitly instantiated in
                 another translation unit
      symbol_type(){}
      ^
    1 warning generated.

The same applies for the basic_symbol's destructor and `clear()`.

* configure.ac (warn_cxx): Add -Wundefined-func-template.
This triggered one failure in the test suite:
* tests/headers.at (Sane headers): here, where we check that we can
compile the generated headers in other compilation units than the
parser's.
Add a variant type to make sure that basic_symbol and symbol_type are
properly generated in this case.
* data/skeletons/c++.m4 (basic_symbol): Inline the definitions of the
destructor and of `clear` in the class definition.

tests/headers.at

@@ -125,7 +125,7 @@ AT_BISON_OPTION_PUSHDEFS([$1])
 AT_DATA_GRAMMAR([input.y],
 [[$1
 %define parse.error verbose
-]AT_VARIANT_IF([], [%union {int integer;}])[
+]AT_VARIANT_IF([%token <int> 'x'], [%union {int integer;}])[
 %code {
 ]AT_PUSH_IF([[
 #if defined __GNUC__ && 7 == __GNUC__