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

[Richard W.M. Jones]

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.