We recently decided to move up to C 99 for ICU4C, but it turns out that Visual Studio does not have an actual C 99 mode. Some language extensions can be turned on, but they are not the same as C 99.
The upshot is that it is easy to write ICU C code on non-Windows platforms that won't compile on Windows.
The feature that bit me was variable declarations that are not at the top of a block.
I have no particular recommendations for what to do - maybe nothing.