Build input/regression/ as doc-build dependency
The build rule for out/offline-root/ tries to copy files from the regtest into the documentation output, and fails if they don't exist.
This unbreaks the install-doc target when run in a www-clean tree, and it drops superfluous files from the regtest output.
rm -rf /tmp/q ; make out=www clean ; make -j4 install-doc DESTDIR=/tmp/q
verified that /tmp/q/usr/local/share/doc/lilypond/html/input/regression/ has just the entry HTML+PNG and the PDF file