doc/doxygen: fix build when --disable-build-doc is enabled

This fixes a regressions introduced in r19804 ("doc: fix doc
generation") when we don't want to build documentation.

doc/doxygen/Makefile.am

@@ -276,9 +276,9 @@ $(DOX_TAG): $(dox_inputs)
 	@$(SED) -i '/\\begin{titlepage}/,$$d' $(DOX_LATEX_DIR)/refman.tex
 	@cat $(top_srcdir)/doc/doxygen/refman.tex >> $(DOX_LATEX_DIR)/refman.tex
 
+if BUILD_DOC
 dist_pdf_DATA = $(DOX_PDF)
 
-if BUILD_DOC
 $(DOX_PDF): $(DOX_TAG) refman.tex
 	@cp $(top_srcdir)/doc/doxygen/chapters/version.sty $(DOX_LATEX_DIR)
 	@cp $(top_srcdir)/doc/doxygen/chapters/*pdf $(DOX_LATEX_DIR)
@@ -302,9 +302,7 @@ $(DOX_PDF): $(DOX_TAG) refman.tex
 	   fi; \
 	done
 	mv $(DOX_LATEX_DIR)/refman.pdf $(DOX_PDF)
-endif
 
-if BUILD_DOC
 CLEANFILES = $(DOX_TAG) starpu_config.h \
     -r \
     $(DOX_HTML_DIR) \