With the 1.1.1 tarball, if a user does 'make && make distcheck',
things pass, but if they do 'make distcheck' after 'make clean',
there is an odd failure:

  GEN      ../../docs/devhelp/index.html
I/O error : Permission denied
I/O error : Permission denied
runtime error: file ../../docs/devhelp/devhelp.xsl line 43 element document
xsltDocumentElem: unable to save to ../../docs/devhelp/libvirt-virterror.html
I/O error : Permission denied
I/O error : Permission denied

This implies that the rules for 'make dist' are missing a
dependency - the generated documentation needs to be up-to-date
before creating the tarball, or else the tarball will be missing
files, where the end user will end up trying to rebuild files in
srcdir, and that fails when srcdir is read-only.

1.1.1 plus this patch now works without issues (other issues have
crept in to 1.1.2-rc1 that prevent 'make distcheck' from working,
but those will be cleaned up in later patches).

* docs/Makefile.am (dist-local): New dependency.
Signed-off-by: NEric Blake <eblake@redhat.com>