I added "libbsd" to the "Requires:" line in lowdown.pc and could compile nix (after making the necessary changes in the nix build system to propagate LOWDOWN_LIBS):
prefix=/usr
exec_prefix=${prefix}
libdir=/usr/lib/x86_64-linux-gnu
includedir=/usr/include
Name: lowdown
Description: simple markdown translator library
URL: https://kristaps.bsd.lv/lowdown
Version: 0.10.0
Requires: libbsd
Libs.private:
Libs: -L${libdir} -llowdown -lm
Cflags: -I${includedir}
