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

Commit 7e3e8ec040b (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>

package/Makefile.in

@@ -284,6 +284,7 @@ TARGET_CONFIGURE_ARGS= \
 	ac_cv_func_malloc_0_nonnull=yes \
 	ac_cv_func_calloc_0_nonnull=yes \
 	ac_cv_func_realloc_0_nonnull=yes \
+	lt_cv_sys_lib_search_path_spec="" \
 	$(BR2_AC_CV_C_BIGENDIAN)
 
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