Certain envs may not support invoking itstool by itself directly as a script
as shebang lines are not supported, such as under cmd.exe shells on Windows,
that are normally used for Visual Studio (and the like, such as clang-cl)
builds.
This will call the corresponding interpreter to invoke itstool when needed, so
that itstool can be properly run, even if shebang lines are not supported by
the env.
This will fix building appstream on Windows using clang-cl, for instance.
Don't assume itstool, msgfmt et al. are just magically on the path.
Normally for commands being processed in build.ninja we'd look up the
program in order to run it. Offer the same guarantee for programs being
passed through an awkward script wrapper.
Chun-wei Fan