c++: workaround portability issue

On some systems (x86_64-pc-solaris2.11), with Developer Studio 12.5's
CC, we get:

    ".../include/CC/Cstd/vector.cc", line 127: Error: Cannot assign const yy::parser::stack_symbol_type to yy::parser::stack_symbol_type without "yy::parser::stack_symbol_type::operator=(const yy::parser::stack_symbol_type&)";.
    ".../include/CC/Cstd/vector", line 475:     Where: While instantiating "std::vector<yy::parser::stack_symbol_type>::__insert_aux(yy::parser::stack_symbol_type*, const yy::parser::stack_symbol_type&)".
    ".../include/CC/Cstd/vector", line 475:     Where: Instantiated from non-template code.
    1 Error(s) detected.

Don't expect __cplusplus to be always defined.  If it's not, consider
this is C++98.

Reported by Nelson H. F. Beebe.

* data/c++.m4, data/lalr1.cc, examples/c++/variant.yy, tests/local.at,
* tests/testsuite.h:
An undefined __cplusplus means pre C++11.

tests/local.at

@@ -912,14 +912,14 @@ m4_define([AT_REQUIRE_CXX_VERSION],
 [AT_DATA([check.cc],
 [[int main ()
 {
-#if __cplusplus < ]m4_case([$1],
-                           [98], [199711],
-                           [03], [199711],
-                           [11], [201103],
-                           [14], [201402],
-                           [17], [201703],
-                           [2a], [201709],
-                           [m4_fatal([$0: invalid arguments: $@])])[
+#if !defined __cplusplus || __cplusplus < ]m4_case([$1],
+    [98], [199711],
+    [03], [199711],
+    [11], [201103],
+    [14], [201402],
+    [17], [201703],
+    [2a], [201709],
+    [m4_fatal([$0: invalid arguments: $@])])[
   return 77;
 #else
   return 0;

tests/testsuite.h

@@ -7,7 +7,7 @@
 /* In C++ pre C++11 it is standard practice to use 0 for the null
    pointer.  But GCC -std=c++98 with -Wzero-as-null-pointer-constant
    warns about this.  Warning introduced in GCC 4.7. */
-#if defined __cplusplus && __cplusplus < 201103L
+#if !defined __cplusplus || __cplusplus < 201103L
 # if defined __GNUC__ && ! defined __clang__ && 407 <= __GNUC__ * 100 + __GNUC_MINOR__
 #  pragma GCC diagnostic ignored "-Wzero-as-null-pointer-constant"
 # endif