GNUMakefile: install libstdbuf

[Ecordonnier]

Use external libstdbuf.so when building with make, as embedding the library
is only needed to work around "cargo install" limitations with shared libraries.

Also change default installation directory to /usr/local/libexec/coreutils/ for consistency
with GNU coreutils

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[oech3]

Is this trying to installing /usr/lib/libstdbuf.so? GNU Coreutils's default is /usr/lib/coreutils/libstdbuf.so if it is built with --libexecdir=/usr/lib.

mkdir and cp is replaced with install -Dm644

[Ecordonnier]

Is this trying to installing /usr/lib/libstdbuf.so? GNU Coreutils's default is /usr/lib/coreutils/libstdbuf.so if it is built with --libexecdir=/usr/lib.

mkdir and cp is replaced with install -Dm644

• I added install -m755
• install -Dm755 does not work because of using a wildcard (libstdbuf*) to handle .so vs .dylib on macos, so it would need to be separated line "install -D". I kept mkdir -p for consistency with the other mkdir calls in the same code block.
• the default of GNU coreutils is to be /usr/local/libexec/coreutils/libstdbuf.so:

ecordonnier@LC49NTQ3:~/dev/gnu-coreutils$ make install DESTDIR=./foobar
ecordonnier@LC49NTQ3:~/dev/gnu-coreutils$ find foobar/ -name libstdbuf*
foobar/usr/local/libexec/coreutils/libstdbuf.so

I changed the code to use the same directory in uutils-coreutils

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[Ecordonnier]

This PR is ready to be reviewed. I am not working on it any more.