x11/xnotify: respect LDFLAGS

$ CFLAGS=-fsanitize=address LDFLAGS=-fsanitize=address make
[...]
cc -o xnotify xnotify.o -L/usr/local/lib -L/usr/local/lib -lfontconfig -lXft -lX11 -lXinerama -lImlib2
ld: error: undefined symbol: __asan_report_load8

referenced by xnotify.c:51

xnotify.o:(usage)

referenced by xnotify.c:1082

xnotify.o:(main)

referenced by xnotify.c:1082

xnotify.o:(main)

referenced 172 more times