[Libguestfs] [PATCH] configure: Move language binding detection to separate files.

This commit starts to split our massive, monolithic configure.ac file into smaller files, using the m4_include mechanism to combine them. I don't know if we should really do this, so I'm open to comments about it. However: - Our configure.ac script is 1800+ lines long, and that's pretty long. - configure.ac lacks structure; splitting it up might improve that. - From what I read, m4_include is a simple include device, so this shouldn't break anything (https://autotools.io/autoconf/macros.html). Rich.