meson: Support system-provided bestsource

[al3xtjames]

Meson unconditionally uses the wrapped bestsource dependency when the bestsource option is enabled. Update meson.build to support using a system-provided version of bestsource when it is present.

[arch1t3cht]

Thanks for the PR (and sorry for taking so long) - Three comments:

1. Do you know of a distribution where this would actually be used? vapoursynth-bestsource is a system package on arch linux, but since it installs into /usr/lib/vapoursynth I don't think it'd get picked up by pkg-config. If not that's not a problem, I'm just curious.
2. The version requirement needs to be >=6.0 now after the latest bump.
3. Normally you could simplify the if-else logic by instead just doing dependency('bestsource', fallback: ['bestsource', 'bestsource_dep'], default_options: [...]). (Or is there some specific reason why you did not do this?) However, this then has the problem that the bestsource wrap currently has a patch to remove the project version declaration, since that uses grep, which is not always available on Windows. So this patch would need to be updated to hardcode a 6.0 version instead. Alternatively, you could keep the current logic and add a comment that this is the reason why you don't simplify it into a simple dependency call with fallback.

[0tkl]

the bestsource wrap currently has a patch to remove the project version declaration, since that uses grep, which is not always available on Windows.

Speaking of this, as a sidenote, grep is distributed along with Git for Windows and is stored in C:\Program Files\Git\usr\bin or C:\Users\USERNAME\AppData\Roaming\Git\usr\bin. I think we can safely assume that anyone with Git installed on Windows can compile bestsource without the patch.

[al3xtjames]

No worries about the delay.

1. I use this patch in a Nix derivation for this Aegisub fork. I also wrote one for bestsource, though neither of these are in nixpkgs. I actually haven't tested if this compiles on NixOS but it does work on macOS.
2. Thanks for pointing this out. I rebased the patch and updated the version.
3. When I originally made this change I didn't realize that default_options could be passed to dependency for fallbacks, which is why I used an if-else. I updated the patch to use fallback and updated the patch in the bestsource wrap to hardcode the verison to 6.0.