Add cangjiecommon.h to put commonly used stuff into it

Currently, it includes necessary definitions for
1. calling the library from C++
2. Dynamic linking on MSVC
Merge branch 'extern-c-dllexport' into msvc-proj
Add cangjiecommon.h to libcangjie2.vcxproj
Add libcangjie_derivative_dll
Add Debug-DLL build configuration
Also make it compile on both MinGW and MSVC
Merge branch 'setenv' into msvc-proj
Add the return statement at the end of main to fix warning
Merge branch 'return-statement' into msvc-proj
Implement strtok_r for Windows
Merge branch 'setenv' into msvc-proj