Mk/subdir.mk: Support partial installation

When using the wrapper Makefile (Mk/subdir.mk), allow make install to
work from within a subdir. The solution is a bit of a hack, and contains
some hard-coded assumptions, and only works for lib/ and stuff under
modules/, but that's where 99% of the work is done anyway.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Mk/subdir.mk: When running under a subdir of lib/, work on lib

Directories under lib/ (except for ivykis, where this doesn't apply
anyway) are for organisational purposes only, they are used as part of
libsyslog-ng, built from lib/ itself. So when standing in such a subdir,
pretend we're in lib instead.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>