Automake explicitly recognizes the use of @LIBOBJS@ and @ALLOCA@, and uses this information, plus the list of LIBOBJS files derived from configure.in to automatically include the appropriate source files in the distribution (Chapter 16). These source files are also automatically handled in the dependency-tracking scheme; see the section called “Automatic dependency tracking”.
@LIBOBJS@ and @ALLOCA@ are specially recognized in any _LDADD or _LIBADD variable.