We provide a mean for overriding Cobalt wrapper routines with
user-provided routines.

Each "foo" Cobalt routine shadowing a POSIX service may then appear
as:

__real_foo() => Original POSIX implementation.
__cobalt_foo() => Cobalt implementation.
__wrap_foo() => Weak alias to __cobalt_foo(). May be overriden
by some extension library.

Extension libraries overriding some of the shadowed POSIX routines
shall provide:

* Their own implementation of the POSIX service as __wrap_foo()
* An entry for the "foo" symbol in some override definition file, to
be installed in $(prefix)/lib, and named after the interface. This
file must have the same format than lib/cobalt/cobalt.wrappers.

E.g. a new interface "foo" overriding POSIX symbols should install a
file named $(prefix)/lib/foo.wrappers. This file will be picked
automatically by the xeno-config script when built for the Cobalt
core.

The Cobalt implementation of any overriden "foo" routine is still
callable as __COBALT(foo(args...)). Any call reference to foo() will
branch to the user-defined implementation overriding Cobalt's.

e.g. Library "foo" wants to provide its own implementation of service
sched_yield().

lib/foo
	/foo.wrappers
		...
		--wrap sched_yield
		...

	/pthread.c
		int __wrap_sched_yield(void)
		{
			/* Eventually branch to Cobalt's version. */
			return __COBALT(sched_yield());
		}

A Makefile rule should install lib/foo/foo.wrappers into $(prefix)/lib.

$ cd buildir
$ .../xenomai/configure --with-cobalt-override=.../lib/overlib/overlib.wrappers

NOTE: for pratical reasons, xeno-config and wrap-link.sh scripts
expect all POSIX wrapper files to match the "*.wrappers" glob pattern.