Actually install the documentation with make install

Instead of just its build stamp :)

doc/Makefile.am

@@ -5,23 +5,38 @@ DOCINPUTS = $(top_srcdir)/include/opus.h \
             $(top_srcdir)/include/opus_defines.h \
             $(top_srcdir)/include/opus_types.h
 
-doc_DATA = doxygen-build.stamp
-
 EXTRA_DIST = Doxyfile.in
 
+all-local: doxygen-build.stamp
+
+
 if HAVE_DOXYGEN
 doxygen-build.stamp: Doxyfile $(DOCINPUTS)
 	doxygen
 	touch $@
+
+install-data-local:
+	for f in `find html -type f \! -name "installdox"`; do	\
+		$(INSTALL_DATA) -D $$f $(DESTDIR)$(docdir)/$$f;	\
+	done
+
+	$(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 -rf html
-	-rm -rf latex
-	-rm -rf man
-	-rm doxygen-build.stamp
+	$(RM) -r html
+	$(RM) -r latex
+	$(RM) -r man
+	$(RM) doxygen-build.stamp
+
+uninstall-local:
+	$(RM) -r $(DESTDIR)$(docdir)/html
+	$(RM) $(DESTDIR)$(mandir)/man3/opus_*.3 $(DESTDIR)$(mandir)/man3/opus.h.3