Makefile: allow user set LDFLAGS for libbtrfs.so as well

Detected by gentoo's QA checker:

 * QA Notice: Files built without respecting LDFLAGS have been detected
 *  Please include the following list of files in your report:
 * /usr/lib/libbtrfs.so.0.1

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Makefile

@@ -93,7 +93,8 @@ version.h:
 
 $(libs_shared): $(libbtrfs_objects) $(lib_links) send.h
 	@echo "    [LD]     $@"
-	$(Q)$(CC) $(CFLAGS) $(libbtrfs_objects) $(lib_LIBS) -shared -Wl,-soname,libbtrfs.so -o libbtrfs.so.0.1
+	$(Q)$(CC) $(CFLAGS) $(libbtrfs_objects) $(LDFLAGS) $(lib_LIBS) \
+		-shared -Wl,-soname,libbtrfs.so -o libbtrfs.so.0.1
 
 $(libs_static): $(libbtrfs_objects)
 	@echo "    [AR]     $@"