GitLab

Platform specific header files get installed.  This allows plugins to be built with just an OpenMM installation, not a full source tree.

- MingW, where "WIN32" is set by CMake but the compiler is not Microsoft's (should use _MSC_VER) in that case, and 
- in gcc 4.4 which is fussy about some issues that earlier versions were silent about, such as passing a string constant to a "char*" rather than a "const char*".
I also fixed a syntax error in CMakeLists that showed up in the OpenMM workshop for some unknown reason.

Brook is not currently building correctly; I think it has not yet been fully converted to the getTime() interface needed for variable step integrators.

1) Added DLL export macros to many classes; enough to get tests to compile
2) Cosmetic changes to reduce warnings in Visual Studio:
   a) change many "for (int i ="... to "for (size_t i ="... to avoid unsigned mismatch warnings
   b) included many explicit static_cast<OpenMMReal>() casts to document places where double->float conversion warnings occur