Document why `Build.exe` passes a `DCC_DcuOutput` parameter to `msbuild.exe`

[Jeroen Wiert Pluimers]

Even though the various .dproj files all specify a dccuouputpath entry, Build.exe passes a fresh one to msbuild.exe, for instance:

"C:\Program Files (x86)\Embarcadero\RAD Studio\12.0\bin\rsvars.bat"
"Packages\DelphiXE5\Spring4D.groupproj"
"Config=Release"
"Platform=iOSDevice"
"DCC_DcuOutput=C:\Users\Developer\Versioned\Spring4D\Library\DelphiXE5\iOSDevice\Release"
pause

Why is this done?

[Stefan Glienke]

Because I coded it that way in order to fix issue #24. Is there a problem with that solution?