In the generated C wrappers, instantiate std::string arguments from
'const char*' by passing it straight to the std::string constructor rather than
through a reinterpret_cast.  This solves segfaults when trying to use the OpenMM
C-API (which is auto-generated from the generate*Wrappers.py scripts) when used
with gcc/g++.  clang and clang++ seem to accept this (perhaps why it was not
caught before).

After these changes, the OpenMM-Tinker binding works again with the current git
version of OpenMM.
Merge pull request #319 from swails/master

Fix segfaults in GCC when using the C-API