Since Modules/ossaudiodev.c has the following line (which is not surrounded by any conditional #ifdef sort of things):

#include <sys/soundcard.h>

This means that the OSS plugin will only build on a system that has this header file, so it is safe to check for on any system.  How can the Module code build if the header can't be found?

The code will only compile if the CFLAGS used when building the
module is setup to include /usr/include or /usr/local/include, but
this is normally taken care of by default.

If users need to specify their OSS headers are in some weird 
non-standard location, then they can use CFLAGS or other Makefile mechanisms to specify the compiler arguments they would then need to build anything that uses their non-standard OSS.

Or am I just confused by your question?