[sylpheed:37260] autoconf and automake instructions in INSTALL

Hi!

I miss autoconf and automake instructions in INSTALL, right
before ./configure and make instructions.

I am a software engineer who has worked almost always with Linux and
Unix, but have only needed to generate manually configure without
instructions once or twice, and I don't remember, so I would appreciate
either confirming when INSTALL is completed on the git repository.

Thanks in advance, and best regards to all.