"conditional +=" doesn't work as suggested by documentation

[Richard Brooksby]

At <http://sources.redhat.com/automake/automake.html> it says:
> 7.1.3.2 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 (see 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

The former works, the latter does not.  I'm running automake 1.6.3 on  
Mac OS X 10.4.  Try putting the attached minimal files in a directory  
and running "aclocal" followed by "automake".

configure.ac
Description: Binary data

Makefile.am
Description: Binary data

I get output like this:

address@hidden automake
Makefile.am:4: hello_SOURCES was already defined in condition TRUE,  
which implies condition LINUX_TRUE
  hello_SOURCES (User, where = Makefile.am:4) +=
  {
    TRUE => hello-common.c
  }
Makefile.am:4: hello_SOURCES was already defined in condition TRUE,  
which implies condition LINUX_FALSE
  hello_SOURCES (User, where = Makefile.am:4) +=
  {
    TRUE => hello-common.c
    LINUX_TRUE => hello-linux.c
  }
Makefile.am:4: warning: automake does not support conditional  
definition of hello_SOURCES in hello_SOURCES
Makefile.am:4: warning: automake does not support conditional  
definition of hello_SOURCES in hello_SOURCES
Use of uninitialized value in concatenation (.) or string at /usr/bin/ 
automake line 8449.
: am_hello_OBJECTS was already defined in condition LINUX_TRUE, which  
is implied by condition TRUE
  am_hello_OBJECTS (Automake, where = undefined) =
  {
    LINUX_TRUE => hello-common.$(OBJEXT) hello-linux.$(OBJEXT)
  }
Use of uninitialized value in concatenation (.) or string at /usr/bin/ 
automake line 8449.
: am_hello_OBJECTS was already defined in condition TRUE, which  
implies condition LINUX_FALSE
  am_hello_OBJECTS (Automake, where = undefined) =
  {
    TRUE => hello-common.$(OBJEXT)
    LINUX_TRUE => hello-common.$(OBJEXT) hello-linux.$(OBJEXT)
  }
address@hidden