Add copy of opus library in deps/third_party.

opus/doc/Makefile.am

+## Process this file with automake to produce Makefile.in
+
+DOCINPUTS = $(top_srcdir)/include/opus.h \
+            $(top_srcdir)/include/opus_multistream.h \
+            $(top_srcdir)/include/opus_defines.h \
+            $(top_srcdir)/include/opus_types.h \
+            $(top_srcdir)/include/opus_custom.h \
+            $(top_srcdir)/doc/header.html \
+            $(top_srcdir)/doc/footer.html \
+            $(top_srcdir)/doc/customdoxygen.css
+
+EXTRA_DIST = customdoxygen.css Doxyfile.in footer.html header.html opus_logo.svg
+
+
+if HAVE_DOXYGEN
+
+all-local: doxygen-build.stamp
+
+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 -name opus_*.3 \
+                  -exec $(INSTALL_DATA) \{} $(DESTDIR)$(mandir)/man3 \;
+
+clean-local:
+        $(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
+
+endif