buildsys: Fix O= PREFIX= case

make sure that something like:
$ rm -rf /dev/shm/o/* /dev/shm/o/.conf* /dev/shm/DELME
$ time (make defconfig O=/dev/shm/o 2>&1 >& /dev/null && make \
install_dev -j O=/dev/shm/o PREFIX=/dev/shm/DELME 2>&1 >& /dev/null)
works

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Makefile.in

@@ -305,7 +305,7 @@ endif
 install_dev: install_headers install_runtime | $(PREFIX)$(DEVEL_PREFIX)$(MULTILIB_DIR)
 	-$(INSTALL) -m 644 $(top_builddir)lib/*.[ao] $(PREFIX)$(DEVEL_PREFIX)$(MULTILIB_DIR)/
 ifeq ($(HAVE_SHARED),y)
-	for i in `find $(top_builddir)lib/ -type l -name 'lib[a-zA-Z]*.so' | \
+	for i in `cd $(top_builddir) && find lib/ -type l -name 'lib[a-zA-Z]*.so' | \
 	$(SED) -e 's/lib\///'` ; do \
 		$(LN) -sf $(RUNTIME_PREFIX_LIB_FROM_DEVEL_PREFIX_LIB)$$i.$(MAJOR_VERSION) \
 		$(PREFIX)$(DEVEL_PREFIX)$(MULTILIB_DIR)/$$i; \
@@ -343,7 +343,7 @@ ifeq ($(DOPIC),y)
 #	# If we build shared libraries then the static libs are PIC...
 #	# Make _pic.a symlinks to make mklibs.py and similar tools happy.
 	if [ -d $(top_builddir)lib ] ; then \
-		for i in `find $(top_builddir)lib/ -type f -name 'lib*.a' | $(SED) -e 's/lib\///'` ; do \
+		for i in `cd $(top_builddir) && find lib/ -type f -name 'lib*.a' | $(SED) -e 's/lib\///'` ; do \
 			$(LN) -sf $$i $(PREFIX)$(DEVEL_PREFIX)$(MULTILIB_DIR)/`echo $$i \
 				| $(SED) -e 's/\.a$$/_pic.a/'`; \
 		done ; \