conditional subdir handling

[Thien-Thi Nguyen]

hi automake folks,

i would like to have

  SUBDIRS = a b c D

but control recursion into D via a configure.in "enable_D" var.  any hint
would be appreciated.  if there's a recommended way, i'll write it up and
submit a doc patch.

the docs (autoconf/automake) don't mention "conditional subdirs" like this,
although they both describe unconditional subdir handling.  a scan through
automake list archives 1-yr back for subject "subdir" did not show anything
relevant.

thi