maint: use correct MIT license.

The GPLv3+ license was a cut-n-paste error.
* lyaml-rockspec.lua (license): Correct license is MIT.
* GNUmakefile, Makefile.am, bootstrap.conf, configure.ac:
Relicense files authored by me under MIT.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>