prevent unnecessary recompilation

[lzy1g1225]

I found that libtins got recompile everytime which drive me crazy when I switched debug/release mode or when I switched from win32/linux. The reason is the cmake generates config.h for those build types into source folder and got the config.h overwritten, which causing the compiler to recompile the whole project. I suggest to generate config.h in binary directory. That make every config.h stay with there own build type.

[lzy1g1225]

Looks like the appveyor build check failed because tests and examples didn't update include path for config.h. Will add this later.