Without the change build on -fno-common toolchains like upstream gcc-10
fails as:
$ nix build -L --impure --expr 'with import ./. {}; ncrack.overrideAttrs (oa: { NIX_CFLAGS_COMPILE = (oa.NIX_CFLAGS_COMPILE or "") + "-fno-common"; })'
...
ld: opensshlib/libopenssh.a(umac128.o):/build/source/opensshlib/umac.c:1184:
multiple definition of `umac_ctx'; opensshlib/libopenssh.a(umac.o):/build/source/opensshlib/umac.c:1184: first defined here