CMake support. Srb2win.rc still needs touchy, whatever.

Also updated CMakeLists.txt, appveyor.yml and this one Xcode project file as usual

This reverts commit d03c5393.

Don't worry, I remembered to update MODVERSION as well this time :)

Core code has too many #define dependencies on interface-specific
defines. This means that it's currently not possible to safely
separate the core and interface code into different contexts. The
core code should be refactored to accomadate for this because we
should not have any interface-specific code in core in the first
place.

This reverts the static library SRB2Core from a7135094 and instead
adds the core sources to the SRB2SDL2 target directly.

So frustrating...

In-source builds are pain and suffering and I am going to stop people
from doing them to save them that pain.

Only when actually making the debug target though!

this is done as a POST_BUILD step