This allows the following in post the c++11 world:
```
tclap::ValueConstraint<std::string> constraints({"foo", "bar"});
```
instead of
```
std::vector<std::string> constraint_choices({"foo", "bar"});
tclap::ValueConstraint<std::string> constraints(constraint_choices);
```
Code would not compile with HAVE_STRSTREAM defined.
Code would not compile without HAVE_CONFIG_H defined (unless
-DHAVE_STRSTREAM or -DHAVE_SSTREAM was specified).
Code would not compile with HAVE_CONFIG_H defined if config.h did not
also define HAVE_SSTREAM.
This patch moves annoying logic for sstream/strstream logic to it's
own file and fixes the above mentioned issues.
https://sourceforge.net/p/tclap/bugs/23/
