The gl subdirectory contains our local versions of gnulib module
implementations, so move m4/acinclude.m4 and m4/dospaths.m4 there.
* gl/modules/make-macros: Create a new module to handle the macros.
* bootstrap.conf: Add the new module.
* configure.ac: Macro invocation is moved to make-macros.
* m4/.gitignore: Delete unnecessary ignore file: m4 is empty.
* .gitignore: Add m4/ as an ignored directory.
