docs: Create html documentation even if XHTML1 DTD is not available to validate

docs: Create html documentation even if XHTML1 DTD is not available to validate

On a Debian system lacking the w3c-dtd-xhtml package the build fails
with:

        $ make -C docs/ formatcaps.html
        make: Entering directory '/local/scratch/ianc/devel/libvirt.git/docs'
        Generating formatcaps.html.tmp
        I/O error : Attempt to load network entity http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd
        formatcaps.html.in:2: warning: failed to load external entity "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
        C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
                                                                                       ^
        I/O error : Attempt to load network entity http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd
        ../docs/sitemap.html.in:2: warning: failed to load external entity "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
        C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
                                                                                       ^
        missing XHTML1 DTD
        rm formatcaps.html.tmp
        make: Leaving directory '/local/scratch/ianc/devel/libvirt.git/docs'
        $ ls docs/formatcaps*
        docs/formatcaps.html.in

https://www.redhat.com/archives/libvir-list/2009-November/msg00413.html
suggests that the XHTML1 DTD should not be a hard requirement and the
docs should be generated but not validated if it is not available.

Therefore when the DTD is not available arrange for the .html.tmp file
to be propagated to the .html output.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Daniel Veillard <veillard@redhat.com>