bug#42464: v1.6.0 fails to build on OS X 10.11: "error: use of undeclared identifier 'CLOCK_MONOTONIC'"

[Leon Klingele]

Package: adns

Steps to reproduce:

```sh
$ brew install adns
# [..]
==> make
set -e; for d in src  client regress; do /Applications/Xcode.app/Contents/Developer/usr/bin/make -C $d all; done
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o types.o types.c
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o event.o event.c
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o query.o query.c
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o reply.o reply.c
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o general.o general.c
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o setup.o setup.c
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o transmit.o transmit.c
clang -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign -Wno-unused-value  -I. -I.    -c -o parse.o parse.c
event.c:158:25: error: use of undeclared identifier 'CLOCK_MONOTONIC'
  int r = clock_gettime(CLOCK_MONOTONIC,&ts);
                        ^
1 error generated.
make[1]: *** [event.o] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [all] Error 2
```