Platform dependent source files: what is best?

[ucomesdag]

I'm writting somthing that runs on windows/mac osx/linux.
If got one set of source filles that is dependent on the platform ho do i best include them?

win32 {
	#LIBS +=  ./WinMM.Lib
	HEADERS += ./windows/midiIO.h
	SOURCES += ./windows/midiIO.cpp
	message(Including Windows specifique headers and sources...)
}
unix {
	HEADERS += ./linux/midiIO.h
	SOURCES += ./linux/midiIO.cpp
	message(Including Linux specifique headers and sources...)
}
macx {
	HEADERS += ./macosx/midiIO.h
	SOURCES += ./macosx/midiIO.cpp
	message(Including Mac OS X specifique headers and sources...)
}

win32 {
	#LIBS +=  ./WinMM.Lib
	HEADERS += ./windows/midiIO.h
	SOURCES += ./windows/midiIO.cpp
	message(Including Windows specifique headers and sources...)
}
unix {
	HEADERS += ./linux/midiIO.h
	SOURCES += ./linux/midiIO.cpp
	message(Including Linux specifique headers and sources...)
}
macx {
	HEADERS += ./macosx/midiIO.h
	SOURCES += ./macosx/midiIO.cpp
	message(Including Mac OS X specifique headers and sources...)
}

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If I put it like this in their own sub dir. it complains that it cant find #include "midiIO.h" when compiling so I have to include with the path to the sub dir. #include "windows/midiIO.h" so I have to change the include on every platform....?????

I'm a bit lost....
....any advise???