Try adding one or more of the following to your CFLAGS:
-no-Werror
-no-pedantic -no-pedantic-errors
-std=gnu89
I don't know whether any of these will actually work; these just looked (based on the cpp options doc) like the most likely candidates to solve your problem.
Another thing that you can do is look for a file named "spec" somewhere under "/lib/gcc-lib" ("find /lib/gcc-lib -name spec -print") on both your Debian and non-Debian systems, and compare them to each other.
That may give you a clue as to what option you need to set or disable to get it working under Debian.
If none of these work, you can work around the bug by "#undef"ing id386 in snd_mix.c.
This will compile the C versions of the routines, rather than the assembly language versions.
The result will be slower, but at least it will compile.