Add Meson

[meator]

I do not expect this PR to be merged. As I understand it, XBPS is not under active development. Features & bugfixes are accepted when they are needed, active development of features is not the focus of XBPS. The current configure system works, so I don't think this PR will be merged.

I have discussed my intention of introducing Meson to XBPS in #xbps some time ago.

I wanted to take a look at the internals of XBPS. I wanted to begin at the build system level to become familiar with XBPS's structure. This PR is a result of that. I also wanted to review the current build system. I have found #598 thanks to this. I have found additional issues while revisiting this now, but they are too inconsequential to make a PR for them.

I also find Meson more practical to work with both as an user and as a developer. Out of the box compile_commands.json support, ccache enabled by default, easy management of sanitizers etc. can be useful for development. Having an alternative build system can be an useful resource when debugging certain issues.

Pros:

• out-of-source builds
• simpler syntax
• ease of use
• 50% faster than current system (but configure time is negligible for both)
• nice compile_commands.json support
• #350 is easy to adapt to this PR
• more "correct" in certain areas than current system (uninstall target isn't broken, flag & feature checking is done through Meson's functions, which are more robust, some feature checks are better)
• dependencies can be resolved through Meson's Wrap system
• Meson's verbose output, error messages and logfiles are miles ahead of the current system

Cons:

• this PR requires Meson >=1.3.0 (it shouldn't be that hard to support an earlier version though)

[meator]

As stated above, I don't think that this PR has much of a future. But if there is interest in introducing Meson to XBPS or if this could be discussed further, I'd be willing to continue working on it. This PR is production ready (except for missing tests and git revisions which both require modifying files).

Fully static XBPS builds can be set up like this under Musl:

LDFLAGS=-static meson setup build -Dprefer_static=true --force-fallback-for=zlib -Ddefault_library=static -Dprefix=/usr