build: Fixed the pre-processor separator in Visual Studio project files

A left over from the VC6 project files, so mainly cosmetic in Visual
Studio .NET as it can handle both comma and semi-colon characters for
separating multiple pre-processor definitions.

However, the IDE uses semi-colons if the value is edited, and as such,
this may cause problems in future for anyone updating the files or
merging patches.

Used the Visual Studio IDE to correct the separator character.