Add /utf-8 to MVSC

[NyakoFox]

Changes:

This PR adds the /utf-8 flag to the CMakeLists.txt file. Without it, Visual Studio might choke on the UTF-8 characters in the source.

Legal Stuff:

By submitting this pull request, I confirm that...

• My changes may be used in a future commercial release of VVVVVV
• I will be credited in a CONTRIBUTORS file and the "GitHub Friends"
  section of the credits for all of said releases, but will NOT be compensated
  for these changes unless there is a prior written agreement

[leo60228]

Have you confirmed what version of MSVC this flag was introduced in?

[NyakoFox]

Added a check.