generate c++ compatible code

Thanks to Sweta Seethamraju for providing us the patch!

Patch summary that Sweta sent to Csmith mailinglist:

== quote ==
Please find attached a patch to make csmith output code that can be compiled as C++. The main issues I have handled in this patch are:
1. conversion from (void*)0 to NULL.
2. typecast between signed and unsigned int pointers.
== quote end ==

To generate C++ compatible code, use the command line below:

csmith --lang-cpp --no-consts --no-volatiles