build: avoid corrupting / in RHEL 5

I noticed this while building from libvirt.git on RHEL 5.6:

Generating internals/command.html.tmp
mkdir: cannot create directory `/internals': Permission denied

If I had been building as root instead, this pollutes /.

Older autoconf lacks $(builddir), but it is rigorously equal to '.'
in newer autoconf, so we could use '$(MKDIR_P) internals' instead.

However, since internals/command.html is part of the tarball, we
_already_ build it in $(srcdir), not $(builddir) during VPATH
builds, so the mkdir is wasted effort!

* docs/Makefile.am (internals/%.html.tmp): Drop unused mkdir.

docs/Makefile.am

@@ -127,7 +127,6 @@ hvsupport.html.in: $(srcdir)/hvsupport.pl $(srcdir)/../src/libvirt_public.syms \
 internals/%.html.tmp: internals/%.html.in subsite.xsl page.xsl sitemap.html.in
 	@if [ -x $(XSLTPROC) ] ; then \
 	  echo "Generating $@"; \
-	  $(MKDIR_P) "$(builddir)/internals"; \
 	  name=`echo $@ | sed -e 's/.tmp//'`; \
 	  $(XSLTPROC) --stringparam pagename $$name --nonet --html \
 	    $(top_srcdir)/docs/subsite.xsl $< > $@ \