add thread-related CFLAGS needed on some platforms

Makefile

@@ -1,10 +1,11 @@
-CFLAGS  := -std=c99 -Wall -O2
+CFLAGS  := -std=c99 -Wall -O2 -D_REENTRANT
 LIBS    := -lpthread -lm
 
 TARGET  := $(shell uname -s | tr [A-Z] [a-z] 2>/dev/null || echo unknown)
 
 ifeq ($(TARGET), sunos)
-	LIBS += -lsocket
+	CFLAGS += -D_PTHREADS
+	LIBS   += -lsocket
 endif
 
 SRC  := wrk.c aprintf.c stats.c units.c ae.c zmalloc.c http_parser.c tinymt64.c