Conditional subdirectory configuration and distclean

[Matt Fago]

I have a project with a large number of subdirectories, only a few of 
which need to be built at any one time. Automake successfully builds 
only the desired directories using AM_CONDITIONAL.

However, ideally I would like to only run configure in the directories 
to be built. Configuration can take quite a while, and I'm not sure I 
want to use a cache (if that would even help much). It is possible to 
set AC_CONFIG_SUBDIRS conditionally such as with:

if test "opt" = true; then
  AC_CONFIG_SUBDIRS[opt]
else
  AC_CONFIG_SUBDIRS[bin]
fi
This does configure as desired, but the distclean target -- which uses 
DIST_SUBDIRS instead of SUBDIRS -- fails in the directories not 
configured (obviously -- no Makefile).

In this case, I would want distclean to attempt to run in all 
DIST_SUBDIRS, but issue a warning and continue if a Makefile is not 
found. As it is distclean issues an error and exits. I'd live with being 
able to set distclean to use SUBDIRS, but this could lead to an 
incomplete distclean in some cases.

Any suggestions?

 - Matt