Get binaries building on MacOS.

gtests/net/packetdrill/Makefile.Darwin

-packetdrill-ext-libs := -lpthread -lpcap
+packetdrill-ext-libs := -pthread -lpcap
+LDFLAGS =
 include Makefile.common

gtests/net/packetdrill/Makefile.common

 all: binaries
 
 CFLAGS = -g -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
+LDFLAGS ?= -static
 
 parser.o: parser.y
 	bison --output=parser.c --defines=parser.h --report=state parser.y
@@ -35,7 +36,7 @@ packetdrill-lib := \
 packetdrill-objs := packetdrill.o $(packetdrill-lib)
 
 packetdrill: $(packetdrill-objs)
-	$(CC) -o packetdrill -g -static $(packetdrill-objs) $(packetdrill-ext-libs)
+	$(CC) -o packetdrill -g $(LDFLAGS) $(packetdrill-objs) $(packetdrill-ext-libs)
 
 test-bins := checksum_test packet_parser_test packet_to_string_test
 tests: $(test-bins)