mac os x: use awk selected by build system rather than first in path

Prior to this patch, the ChangeLog generation was hard coded to use
"awk", when it should have been using the AWK variable set by our
build system.

This breaks compilation on a newly installed OS X system, where the
default path has the Mac (non GNU) awk in the default search PATH
before any installed GNU awk (gawk).

docs/Makefile.am

@@ -88,7 +88,7 @@ api: libvirt-api.xml libvirt-refs.xml
 web: $(dot_html) html/index.html devhelp/index.html
 
 ChangeLog.xml: ../ChangeLog ChangeLog.awk
-	awk -f ChangeLog.awk < $< > $@
+	$(AWK) -f ChangeLog.awk < $< > $@
 
 ChangeLog.html.in: ChangeLog.xml ChangeLog.xsl
 	@if [ -x $(XSLTPROC) ] ; then \