BUILD: Fix gcc's warnings

CMakeLists.txt

inc/lektor/queue.h

src/base/bufferfd.c

src/base/cmd.c

src/base/commands.c

src/base/common.c

src/base/config.c

src/base/json.c

src/base/launch.c

src/base/logfile.c

src/base/os.c

src/base/reg.c

src/base/segv.c

src/base/stack.c

src/base/uri.c

src/database/cache.c

src/database/config.c

src/database/find.c

src/database/open.c

src/database/playlist.c