I think the best idea would be to split up your project into a main file and imported file or something like that and just do
?Win32
Import "file_win32_or_linux.bmx"
?Linux
Import "file_win32_or_linux.bmx"
?MacOS
Import "file_macos.bmx"
?
?Win32
Import "file_win32_or_linux.bmx"
?Linux
Import "file_win32_or_linux.bmx"
?MacOS
Import "file_macos.bmx"
?