configure: Disable installer build by default.

Only build installer if "--enable-installer" is passed. In that case only, the
support for Guile-newt becomes mandatory.

* configure.ac: Add --enable-installer argument. Export ENABLE_INSTALLER
conditional according to the argument value.
* gnu/local.mk (GNU_SYSTEM_MODULES): Remove installer modules and build them
only if ENABLE_INSTALLER is set.