librt is not present on Mac. Only link it on Linux builds.

Based on the prior commit introducing linking with the rt lib,
it was added because Linux (ubuntu) needed it for clock_gettime.

Only add -lrt when linking on Linux.

Makefile

@@ -62,7 +62,7 @@ endif
 # only set main compile options if none were chosen
 CFLAGS += -W -Wall -O2 -D$(TARGET_OS) -Iinclude $(COPT) -DUSE_STACK_SIZE=102400
 
-LIBS = -lpthread -lm -lrt
+LIBS = -lpthread -lm
 
 ifdef WITH_DEBUG
   CFLAGS += -g -DDEBUG_ENABLED
@@ -119,7 +119,7 @@ MAIN_OBJECTS = $(addprefix $(BUILD_DIR)/, $(APP_SOURCES:.c=.o))
 LIB_OBJECTS = $(filter-out $(MAIN_OBJECTS), $(BUILD_OBJECTS))
 
 ifeq ($(TARGET_OS),LINUX)
-  LIBS += -ldl
+  LIBS += -lrt -ldl
   CAN_INSTALL = 1
 endif