- Update premake build script for VS2017+ build support.

- Update premake (for VS2019 support).
- Update Xenia build script to detect VS2017+ version.
- Update Xenia premake scripts due to updated premake.
- Fix cc override in Xenia build script.

src/xenia/app/premake5.lua

@@ -39,9 +39,6 @@ project("xenia-app")
     "xenia-vfs",
     "xxhash",
   })
-  flags({
-    "WinMain",  -- Use WinMain instead of main.
-  })
   defines({
     "XBYAK_NO_OP_NAMES",
     "XBYAK_ENABLE_OMITTED_OPERAND",