Fix doc build with doxygen missing

sflphone-common/doc/Makefile.am

  
-SUBDIRS = doxygen dbus-api
+SUBDIRS = dbus-api
+if ENABLE_DOXYGEN
+    SUBDIRS += doxygen
+endif
 
 .PHONY: doc
 doc:

sflphone-common/doc/doxygen/Makefile.am

+if ENABLE_DOXYGEN
+
 SUBDIRS =
 
 docdir = ${prefix}/share/doc/wifidog-@VERSION@
@@ -53,3 +55,5 @@ maintainer-clean-local: clean-local
             -e 's:@-html_footer-@:${HTML_FOOTER}:g'
 	rm $@.tmp3
 	mv $@.tmp4 $@
+
+endif # ENABLE_DOXYGEN