add unit scope for Windows and Registry units

[corneliusdavid]

TL;DR: Win64 projects don't list all needed unit scope names.

When trying to compile the Indy libraries for 64-bit, not all unit scope names were defined in the project (couldn't find the Windows or Registry units). Instead of adding unit scope names to every 64-bit package (like they are in the 32-bit packages) for all versions of Delphi (since XE2 when 64-bit compilers were introduced) or requiring Delphi developers to add them to their own Delphi options, just add compiler directives for the handful of places in the source that need it.

I found "VCL_XE2_OR_ABOVE" already defined and convenient so just used that for both Windows and Registry units.

[rlebeau]

I can merge this PR as a short-term fix. But, as there is already a handful of places in the code where Unit Scope names are being used explicitly but inconsistently, I might suggest expanding on this change to include all RTL units that use Unit Scope Names, like System.SysUtils, System.IOUtils, System.DateUtils, Vcl.xxx, etc. And add a new compiler define to IdCompilerDefines.inc such as USE_UnitScopeNames for better clarity rather than using VCL_XE2_OR_ABOVE.

[corneliusdavid]

That makes good sense. I wouldn't have been surprised if you wanted to change the compiler directive I used in this commit; I found it used for similar purposes elsewhere and it seemed to work so thought if nothing else, I'd bring attention to it. I like USE_UnitScopeNames better.

[rlebeau]

I changed the target branch for this PR, as I'm going to make further changes before merging everything into master.

[corneliusdavid]

If you'd like some help making these changes, let me know how you want to go about it and I can put some time into it.

[rlebeau]

I added additional changes to the add-unit-scope-names branch, let me know what you think or if I missed something. It turned out to be a bit more than I expected.