classes.cpp: Define _XOPEN_SOURCE for isascii()

The GNU libstdc++ headers use isascii(), which is not part of any
POSIX standard. On BSD, this breaks the build. It is however part
of XOPEN, which on Linux is apparently enabled implicitly for C++.

This should fix #649.

bindings/cxx/classes.cpp

@@ -17,6 +17,11 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+/* Needed for isascii(), as used in the GNU libstdc++ headers */
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+
 #include <config.h>
 #include <libsigrokcxx/libsigrokcxx.hpp>