build: Rename "Makefile.inc" Makefile fragments to "local.mk"
This is done to follow a convention used by a lot of GNU packages.
* bin/Makefile.inc: Rename to ...
* bin/local.mk: ... this.
* doc/Makefile.inc: Rename to ...
* doc/local.mk: ... this.
* lib/Automake/Makefile.inc: Rename to ...
* lib/Automake/local.mk: ... this.
* lib/am/Makefile.inc: Rename to ...
* lib/am/local.mk: ... this.
* lib/Makefile.inc: Rename to ...
* lib/local.mk: ... this.
* m4/Makefile.inc: Rename to ...
* m4/local.mk: ... this.
* contrib/t/Makefile.inc: Rename to ...
* contrib/t/local.mk: ... this.
* t/Makefile.inc: Rename to ...
* t/local.mk: ... this. Adapt.
* Makefile.am: Adapt.