build: Fix API docs generation in VPATH build

XSLT allows for two ways of generating the output of transformation.
Either implicit, which xsltproc prints to stdout and can be redirected
to a file using -o file. Or explicit, which means the stylesheet
contains <xsl:document> element which specifies where the output should
be saved. This can be used for generating more files by a single run of
xsltproc and -o directory/ can change the directory where the output
files will be stored.

devhelp.xsl is special in that it combines both options in one
stylesheet, which doesn't work well with -o:

xsltproc --nonet -o ./devhelp/ ./devhelp/devhelp.xsl ./libvirt-api.xml

Outputs 4 *.html files into ./devhelp but then tries to write to
./devhelp/ as a file (hence the I/O error) rather than writing output to
the fifth file devhelp/libvirt.devhelp.

This patch modifies devhelp.xsl so that all files are generated using
<xsl:document> element and -o directory/ can be used to override output
directory where those files are saved.

docs/Makefile.am

@@ -88,7 +88,10 @@ EXTRA_DIST=					\
   sitemap.html.in \
   todo.pl todo.cfg-example
 
-MAINTAINERCLEANFILES = $(dot_html) $(apihtml) $(devhelphtml)
+MAINTAINERCLEANFILES = \
+  $(addprefix $(srcdir)/,$(dot_html)) \
+  $(addprefix $(srcdir)/,$(apihtml)) \
+  $(addprefix $(srcdir)/,$(devhelphtml))
 
 all: web
 
@@ -146,19 +149,20 @@ internals/%.html.tmp: internals/%.html.in subsite.xsl page.xsl sitemap.html.in
 html/index.html: libvirt-api.xml newapi.xsl page.xsl sitemap.html.in
 	-@if [ -x $(XSLTPROC) ] ; then \
 	  echo "Rebuilding the HTML pages from the XML API" ; \
-	  $(XSLTPROC) --nonet $(srcdir)/newapi.xsl $(srcdir)/libvirt-api.xml ; fi
+	  $(XSLTPROC) --nonet -o $(srcdir)/ \
+	  $(srcdir)/newapi.xsl $(srcdir)/libvirt-api.xml ; fi
 	-@if test -x $(XMLLINT) && test -x $(XMLCATALOG) ; then \
 	  if $(XMLCATALOG) '$(XML_CATALOG_FILE)' "-//W3C//DTD XHTML 1.0 Strict//EN" \
 	    > /dev/null ; then \
 	  echo "Validating the resulting XHTML pages" ; \
 	  SGML_CATALOG_FILES='$(XML_CATALOG_FILE)' \
-	  $(XMLLINT) --catalogs --nonet --valid --noout html/*.html ; \
+	  $(XMLLINT) --catalogs --nonet --valid --noout $(srcdir)/html/*.html ; \
 	  else echo "missing XHTML1 DTD" ; fi ; fi
 
 $(addprefix $(srcdir)/,$(devhelphtml)): $(srcdir)/libvirt-api.xml $(devhelpxsl)
 	-@echo Rebuilding devhelp files
 	-@if [ -x $(XSLTPROC) ] ; then \
-	  $(XSLTPROC) --nonet -o devhelp/libvirt.devhelp \
+	  $(XSLTPROC) --nonet -o $(srcdir)/devhelp/ \
 	  $(top_srcdir)/docs/devhelp/devhelp.xsl $(srcdir)/libvirt-api.xml ; fi
 
 python_generated_files = \

docs/devhelp/devhelp.xsl

@@ -13,6 +13,13 @@
   <!-- Build keys for all symbols -->
   <xsl:key name="symbols" match="/api/symbols/*" use="@name"/>
 
+  <xsl:template match="/">
+    <xsl:document xmlns="http://www.devhelp.net/book" href="libvirt.devhelp"
+                  method="xml" encoding="UTF-8" indent="yes">
+      <xsl:apply-templates/>
+    </xsl:document>
+  </xsl:template>
+
   <xsl:template match="/api">
     <book title="{@name} Reference Manual" link="index.html" author="" name="{@name}">
       <xsl:apply-templates select="files"/>