How can I enable option-checking from configure.ac after it being default-disabled?

[Hans-Peter Nilsson]

Hi.  I don't see a way to turn on option-checking after it being
disabled-by-default due to AC_CONFIG_SUBDIRS.  I had expected to
find either an option to AC_DISABLE_OPTION_CHECKING,
e.g. something like AC_DISABLE_OPTION_CHECKING([false]), or
alternatively and more intuitively than two negatives IMHO;
something like AC_OPTION_CHECKING(whichever).  A quick grep for
OPTION_CHECKING in the autoconf git repo didn't find me any new
construct to achieve this.

So, can I have a way to turn *on* option-checking by default
from configure.ac and regardless of AC_CONFIG_SUBDIRS, please?

Why this odd request?  In RAPP,
<http://savannah.nongnu.org/projects/rapp> (feel very welcome to
have a look at its configure.ac), there's the hopefully
self-explanatory:

# The directory test/installtest isn't configured until after
# installation, but to make autoreconf update this directory we
# have to mention it here
if false; then
    AC_CONFIG_SUBDIRS([test/installtest])
fi

And no, test/installtest is not an imported package, just as you
probably understood, a subdirectory with a hello-world-type
test-program to check that installation works properly; that a
program using the RAPP library can be built and run.

This to present *one* hopefully valid reason for turning on
option-checking despite AC_CONFIG_SUBDIRS.  If the above
construct is obsolete (I don't remember where I've seen it, but
I guess either autoconf or automake docs) or there's just a
better way, then I'm all ears (or eyes).  Either way it seems
odd if there's no way to actually make option-checking on by
default, still overridable by the configure option
--enable-option-checking of course.

brgds, H-P