Just a GNU make test with subdir

The subdir foo is considered to be another project, like a git
submodule with its own Makefiles. It produces two output libfoo.a and
libgnufoo.a.

The main Makefile build a "main" program and try to link it against
those libs.

Signed-off-by: Celelibi <celelibi@gmail.com>
Now it compiles

Bug: It doesn't recompile the lib if it's missing and the end target is
up to date.

Signed-off-by: Celelibi <celelibi@gmail.com>
Should work now

Signed-off-by: Celelibi <celelibi@gmail.com>