Improve multicore build

Only run GenerateWixInclude.exe once.
Not thrilled about 64-bit being dependent on 32-bit build but won't be an issue with things as they are now.

Additionally, seems the `GenerateWixInclude` Target could move to the WixBuild.vcxproj.targets since it only processes `.h` files. Could also try adding a `Condition` to the target based on `_GenerateWixIncludeSource` but I forget if the `DependsOn` will be executed if the condition is false. It might not, which would mean the `Condition` would prevent the execution of the target completely.

All of that would reduce the amount the target ran.