make: replace all reference to  Make and gmake with make

On contemporary Linux systems, gmake command is not available, and
GNU Make is installed as /usr/bin/make. Most users are either going to be
running a Linux distribution, or Windows, so 'make' is a better choice than
'gmake'.

Also, to remind poeple that it's 'make' not 'Make', use lowercase when speaking
about the actual command.
make: add introduction
make: add links to projects
make: rename chapter to "automatic variables"

This chapter does not talk about patterns (in the Makefile sense of
the word) at all, so the title was misleading. "Automatic variables" is
a bit technical, but seems to be a better alternative.
make: rename chapter to "patterns"

This chapter talks what is called patterns in Make parlance. "Rules"
is a bit too generic, since "rules" are described in every chapter of this
tutorial.
make: rename chapter to "variables"

That is what GNU Make calls them. The term "variable" is easier to
relate to for e.g. a Python programer. Also, since chapter 2 is now
"automatic variables", this rename makes things more consistent.
make: update justification for not using make VAR=...

Most distributions preconfigure user shells to save history, so the argument
that the commandline has to be retyped is often invalid. Let's deemphesize it,
and focus on the fact that specyfing options on the commandline may leave
no trace and be hard for other people to recreate.
make: add "objectives" sections and subheadings
make: add a paragraph about the default target and Makefile name
make: remove empty chapters and add "futher reading"
Merging @keszybz's changes to the Make lessons
Merge branch 'keszybz-make'