[IMP] integrate less compilation to base makefile

* should not be invoked when just editing documentation (and not
  touching less files) due to make dependency checking
* requires lessc on the path
* not sure it works for subfiles of main less file? May need
  improvements there

Makefile

@@ -6,6 +6,7 @@ SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build
+LESSOPTS      =
 
 # User-friendly check for sphinx-build
 ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)
@@ -19,6 +20,10 @@ ALLSPHINXOPTS   = -d $(BUILDDIR)/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 # the i18n builder cannot share the environment and doctrees with the others
 I18NSPHINXOPTS  = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 
+lessfiles = _extensions/odoo/static/*.less
+_extensions/odoo/static/style.css: $(lessfiles)
+	lessc $(LESSOPTS) $(subst .css,.less,$@) $@
+
 .PHONY: help clean html dirhtml singlehtml pickle json htmlhelp qthelp devhelp epub latex latexpdf text man changes linkcheck doctest gettext
 
 help:
@@ -49,7 +54,7 @@ help:
 clean:
 	rm -rf $(BUILDDIR)/*
 
-html:
+html: _extensions/odoo/static/style.css
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."