meson: add Meson build system support

[bgermann]

Add initial Meson build system configuration for Enchant, replacing the existing Autotools-based build with a modern, fast alternative.

Includes:

• Root meson.build with project setup, dependency checks, and config.h generation
• libgnu/meson.build for gnulib integration with meson-native header processing
• lib/meson.build for shared library (libenchant) build
• src/meson.build for command-line tool build (enchant, enchant-lsmod)
• providers/meson.build for provider module builds (hunspell, nuspell, aspell, etc.)
• tests/meson.build for C++ unit tests with mock providers
• program-tests/meson.build for shell-based program tests
• meson_options.txt for provider feature options

Fixes: #58

[rrthomas]

Many thanks for this, and in particular for getting a non-autotools build system to work with gnulib.

As you can see, I've reopened #58.

I'm sorry that it will take me a while to review, partly because I'm quite busy at present, and partly because I want to consider carefully what to do here. Given the demand for an alternative to autotools, I am keen to accept this build system if possible; but at the same time, I'm reluctant to switch just one of my projects to something different (I use autotools in all my other C-based projects).

The alternatives are:

• Status quo (i.e. don't accept this PR). As I say, I would very much like to accept if at all possible.
• Two build systems. It might work to have the meson build system as a "contrib" component, which I rely on other people to maintain. I would be inclined to offer this alternative if I don't feel able to maintain it myself.
• Switch to meson. This is my preferred option if at all possible. Hence, I want to take a little time to try out the meson build system, and study it to see what maintaining it would involve. I appreciate that meson is widely used, and hence it would probably be good experience for me to have a meson build system in one of my projects.