this patch implements a gnu build system for c_lib. no existing files are touched.
IT is indended to be merged into the git transition repo.

the glue code at toplevel is under construction. but if desired, it may be possible to hook it into the current static toplevel makefile.

(this build system might finally be merged with the python module build system. this is currently not necessary and would just complicate things due to the directory structure.)

CC: @jondo

Component: build

Keywords: build system, c_lib, gsoc

Author: Felix Salfelder

Branch/Commit: u/felixs/14726 @ 510f8b7

Reviewer: François Bissey

Issue created by migration from https://trac.sagemath.org/ticket/14726