Don't bark scary warnings at compile time about no doxygen

We already report whether or not the docs will be built in the summary box
at the end of the configure run.
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 1c34062..7bd13fa 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -11,10 +11,11 @@
 
 EXTRA_DIST = Doxyfile.in opus_logo.svg
 
-all-local: doxygen-build.stamp
-
 
 if HAVE_DOXYGEN
+
+all-local: doxygen-build.stamp
+
 doxygen-build.stamp: Doxyfile $(DOCINPUTS)
 	doxygen
 	touch $@
@@ -27,14 +28,7 @@
 	$(INSTALL) -d $(DESTDIR)$(mandir)/man3
 	cd man && find man3 -type f \
 		  -exec $(INSTALL_DATA) \{} $(DESTDIR)$(mandir)/man3 \;
-else
-doxygen-build.stamp: Doxyfile $(DOCINPUTS)
-	@echo "*** Warning: Doxygen not found; API documentation will not be built."
-	touch $@
-endif
 
-
-# delete doxygen subdirs
 clean-local:
 	$(RM) -r html
 	$(RM) -r latex
@@ -44,3 +38,5 @@
 uninstall-local:
 	$(RM) -r $(DESTDIR)$(docdir)/html
 	$(RM) $(DESTDIR)$(mandir)/man3/opus_*.3 $(DESTDIR)$(mandir)/man3/opus.h.3
+
+endif