Update makefile

Fix issue where librt wasn't linked in, which is sometimes required on certain systems.

makefile

 CC=g++
 CCFLAGS=-O2 -g
-LDFLAGS=-lX11 -lXext
+LDFLAGS=-lX11 -lXext -lrt
 SOURCES=main.cpp x.cpp options.cpp
 OBJECTS=$(SOURCES:.cpp=.o)
 EXECUTABLE=slop