Always add README.html to dist_doc_DATA

rst2html is required to produce README.html from README.rst.  We
include generated README.html to distribution.  And rst2html is not
required when compiling sources in distribution and always README.html
is available.

Makefile.am

@@ -11,10 +11,9 @@ EXTRA_DIST = config.rpath \
 	     makerelease-osx.mk osx-package/DS_Store osx-package/etc/paths.d/aria2c osx-package/etc/manpaths.d/aria2 \
 	     examples/libaria2ex.cc examples/libaria2wx.cc
 
-dist_doc_DATA = README README.rst
+dist_doc_DATA = README README.rst README.html
 
 if HAVE_RST2HTML
-dist_doc_DATA += README.html
 
 README.html: README.rst
 	$(RST2HTML)  $< > README.html