tests: address portability issues about strdup

Reported by Dennis Clarke
<https://lists.gnu.org/r/bug-bison/2021-10/msg00005.html>.
In particular
<https://lists.gnu.org/r/bug-bison/2021-10/msg00023.html>.

* doc/bison.texi, examples/c/glr/c++-types.y,
* examples/c/bistromathic/parse.y tests/testsuite.h:
Define _XOPEN_SOURCE to 600, to get a strdup that works on Solaris.

* tests/glr-regression.at: Use strdup freely.

examples/c/glr/c++-types.y

@@ -53,6 +53,10 @@
 
 %code
 {
+  /* Portability issues for strdup. */
+#ifndef _XOPEN_SOURCE
+# define _XOPEN_SOURCE 600
+#endif
 
 #include <assert.h>
 #include <ctype.h>