9.1.3 Conditional compilation of sources

You can't put a configure substitution (e.g., `@FOO@') into a `_SOURCES' variable. The reason for this is a bit hard to explain, but suffice to say that it simply won't work. Automake will give an error if you try to do this.

Fortunately there are two other ways to achieve the same result. One is to use configure substitutions in _LDADD variables, the other is to use an Automake conditional.