Enable 'make docs' only if configured with '--enable-docs'.

./configure --enable-docs checks for asciidoc and a2x
release.sh: renamed configure.in -> configure.ac
release.sh: updated to use '--enable-docs' in configure
release.sh: don't forget to push as well as push tags
Release 0.13.7.4.
Updated version to 0.13.7.5.dev
release.sh: force directory removal