If Error.h was loaded before windows.h on windows compilers, then it
happily defined GetMessage and then GetMessageA and GetMessageB. However,
if the load order was reversed, then GetMessage is a macro that changes
it to GetMessageA or GetMessageB and you get an error about the same
method being declared twice.
Get around the problem by not defining GetMessage if it is already
declared as a macro.
We made this change a while ago to help with completion in IDEs.
(Completion was matching a bunch of wrapper macros that were almost
never used anywhere.) Most of the changes are in comments, but there are
a few bad macro definitions.
