Rewrite "Building" section in INSTALL.md

Rephrase steps in "Building" section of INSTALL.md accurately together
with examples. Bunch of steps and concepts were not clearly specified,
e.g. how and when the build options ought to be used.

Finally, explain the role of autogen.sh correctly. It's just for
jhbuild. A more precise build instructions, on the other hand, help a
developer unfamiliar with the build system.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@iki.fi>