Use "$(MAKE) -C ..." for running make in subdirectories.

This adds announcements on changes of directory, helping see what happens
better.
Link correctly against a non-installed libcurl.
Have the makefile include local.mk if it exists.

This is to allow setting local variables in a persistent way.
Add a .md extension to go/README.
Update the content of go/README.md to Markdown.
Convert go/README to Markdown format.
Remove the nearly useless go/ENV file.
Reformat merkle_tree_go.cc.