A collection of updates and fixes

[Pencilcaseman]

This pull request implements a collection of fixes that I've implemented for my own projects. Hopefully at least some of these are useful and, if not, can be adapted to be merged into the main repository :)

1. Undefine linux if it is defined. This fixes bugs in WSL environments
2. Update ImGUI to the Docking latest version of the Docking branch
3. Remove the forcing of /MT under MSVC compilation -- causes problems with other libraries and linking
4. Set /SUMBYSTEM:WINDOWS under MSVC compilation -- Otherwise the main cinder app definition won't work as it implements WinMain (IIRC), not the default main function, causing compiler errors in MSVC

Hopefully at least some of the changes in this are useful!