Add doxygen to standard build flow, if found

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/Makefile.am

@@ -1,2 +1,21 @@
 dist_doc_DATA = API.md
 EXTRA_DIST    = Doxyfile
+
+if ENABLE_HTML
+pkghtmldir     = $(docdir)/html
+pkghtml_DATA   = html/*
+
+#
+# Doxygen rules from m4/ax_prog_doxygen.m4
+#
+@DX_RULES@
+
+$(pkghtml_DATA): doxygen-doc
+
+all-local: doc
+
+clean-local:
+	-rm -rf html
+
+doc:  doxygen-doc
+endif