Add CMake build instructions (just a MVP). Keep the Autotools build
instructions for the full support to build gspell (for now).

This MVP is sufficient for gedit needs.

It is a first step to migrate from Autotools to a more modern and
portable build system (especially to better support MS Windows).

CMake has been chosen mainly because:
- It's possible to write our own re-usable code (functions etc).
- Once a module builds successfully with CMake, it will always be
  possible to build it in the future (strong backward compatibility
  guarantees).

Most GNOME modules have migrated to Meson instead, but Meson doesn't
support the two points above.

When building the project with the Autotools, config.h.in was
overwritten.

This reverts commit 72617b85.

To replace CMake by Meson.

This reverts commit 914f4225.

To replace CMake by Meson.

Meson already has all the GNOME-specific functions. With CMake the
disadvantage is that it needs to be implemented (note that the Evolution
mail client uses CMake, so it was probably possible to copy the CMake
code from there).

For convenience.

For gspell Meson is easier than CMake because there is already all the
GNOME-specific features.

gspell_get_resource() was exported by mistake with the Autotools build
system. That function is no longer exported with Meson.

This is the only difference catched with:
$ nm -D libgspell-1.so.2.3.2 | grep T

But this requires to bump the soversion.

Set the soversion to a single number: 3. The soversion with three number
components was set to have the same as used with Libtool. But the
Libtool versioning is a bit complicated, so from now on, keep thing
simpler by having the soversion as a single number. This should be
sufficient for all OS distributions. If not, please open a bug report.

Use Meson instead.

It's easier to put variables in xml/gtkdocentities.ent and use them in
*.xml files, instead of having *.xml.in files.

"gspell-1.0" was needed for putting the docs online on gnome.org. But
this has changed.

So, simplify the gtk-doc module name to "gspell-1".

There was an unused declaration: GSPELL_H_INSIDE

HAVE_CONFIG_H is specific to Autotools. Now that Autotools is gone,
simplify the #include.

That way, if config.h is not included by mistake, there is a compilation
error.

This doesn't change the API. gspell_checker_error_get_type() is still
generated.

The output of
$ nm -D libgspell-1.so.3 | grep T
still has the same list of exported symbols.