(automake-1.16.info)Conditional Sources

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8.1.3 Conditional compilation of sources
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You can’t put a configure substitution (e.g., ‘@FOO@’ or ‘$(FOO)’ where
‘FOO’ is defined via ‘AC_SUBST’) 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.

Conditional Compilation using ‘_LDADD’ Substitutions
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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 that 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.ac’:

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

   In this case, the variable ‘HELLO_SYSTEM’ should be replaced by
either ‘hello-linux.o’ or ‘hello-generic.o’, and added to both
‘hello_DEPENDENCIES’ and ‘hello_LDADD’ in order to be built and linked
in.

Conditional Compilation using Automake Conditionals
...................................................

An often simpler way to compile source files conditionally is to use
Automake conditionals.  For instance, you could use this ‘Makefile.am’
construct to build the same ‘hello’ example:

     bin_PROGRAMS = hello
     if LINUX
     hello_SOURCES = hello-linux.c hello-common.c
     else
     hello_SOURCES = hello-generic.c hello-common.c
     endif

   In this case, ‘configure.ac’ should setup the ‘LINUX’ conditional
using ‘AM_CONDITIONAL’ (Note: Conditionals).

   When using conditionals like this you don’t need to use the ‘EXTRA_’
variable, because Automake will examine the contents of each variable to
construct the complete list of source files.

   If your program uses a lot of files, you will probably prefer a
conditional ‘+=’.

     bin_PROGRAMS = hello
     hello_SOURCES = hello-common.c
     if LINUX
     hello_SOURCES += hello-linux.c
     else
     hello_SOURCES += hello-generic.c
     endif