package/Makefile.in: ensure libtool doesn't search host dirs for libraries

Commit 7e3e8ec0 (CFLAGS/LDFLAGS: don't add -I / -L args for STAGING_DIR)
exposed a lingering libtool problem.

Unless instructed otherwise (using -L) libtool will search its built in
system path for libraries, and use those instead if found. The default
search path is '/usr/lib, /lib, /usr/local/lib', which is no good for
cross compilation.

Fix it by setting the system search path to the empty string, effectively
disabling this feature.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>