9.1.3.1 Conditional compilation using _LDADD substitutions

Automake must know all the source files that could possibly go into a program, even if not all the files are built in every circumstance. Any files which are only conditionally built should be listed in the appropriate `EXTRA_' variable. For instance, if `hello-linux.c' or `hello-generic.c' were conditionally included in hello, the `Makefile.am' would contain:

bin_PROGRAMS = hello hello_SOURCES = hello-common.c EXTRA_hello_SOURCES = hello-linux.c hello-generic.c hello_LDADD = @HELLO_SYSTEM@ hello_DEPENDENCIES = @HELLO_SYSTEM@

You can then setup the @HELLO_SYSTEM@ substitution from `configure.in':

... case $host in *linux*) HELLO_SYSTEM='hello-linux.$(OBJEXT)' ;; *) HELLO_SYSTEM='hello-generic.$(OBJEXT)' ;; esac AC_SUBST([HELLO_SYSTEM]) ...

In this case, HELLO_SYSTEM should be replaced by `hello-linux.o' or `hello-bsd.o', and added to hello_DEPENDENCIES and hello_LDADD in order to be built and linked in.