[CMAKE] Fix daemon() detection on OSX

Summary: On BSDs and macOS, the daemon() function is located in stdlib.h.

Test Plan:
On OSX:

cmake -GNinja ..

Check daemon is found.

ninja
./src/bitcoind -daemon

Check the argument is supported and works as expected.

Run the OSX Gitian build and test the -daemon option is supported on
the produced bitcoind binary.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5006