fastd 20 osx build failure

[chenrui333]

👋 running into the build issue with fastd 20 and 21 releases. The error is as follows:

==> ninja -v
[1/67] clang -Isrc/fastd.p -Isrc -I../src -Xclang -fcolor-diagnostics -pipe -Wall -Winvalid-pch -Wextra -std=c99 -O3 -D_GNU_SOURCE __APPLE_USE_RFC_3542 -MD -MQ src/fastd.p/main.c.o -MF src/fastd.p/main.c.o.d -o src/fastd.p/main.c.o -c ../src/main.c
FAILED: src/fastd.p/main.c.o 
clang -Isrc/fastd.p -Isrc -I../src -Xclang -fcolor-diagnostics -pipe -Wall -Winvalid-pch -Wextra -std=c99 -O3 -D_GNU_SOURCE __APPLE_USE_RFC_3542 -MD -MQ src/fastd.p/main.c.o -MF src/fastd.p/main.c.o.d -o src/fastd.p/main.c.o -c ../src/main.c
clang: error: no such file or directory: '__APPLE_USE_RFC_3542'

Full error log in here, https://github.com/Homebrew/homebrew-core/runs/1324700334 relates to https://github.com/Homebrew/homebrew-core/pull/62644