Added meson file, to declare a dependency.

[jormundgand]

Added a meson build file, so that doctest can be used as a subproject from git.

The build file doesn't actually build the project, but just defines a dependency.

[onqtam]

Thanks for the contribution!

Do you think it's possible to get the version programmatically from https://github.com/onqtam/doctest/blob/master/scripts/version.txt ?

[jormundgand]

Meson is very big on not being able to change anything programmatically and the project definition has to be the first thing to be defined -- that makes it a tad difficult to do it. Personally, I'm not that worried about the version lagging a bit. Since you're already storing the result of the build in the repo, I guess you could construct the meson build file in the same go. Alternatively, you could just remove the version from the file.

[onqtam]

I'll perhaps generate it