b2sum: Fix build on darwin

Fixes #272923.

pkgs/tools/security/b2sum/default.nix

@@ -23,6 +23,11 @@ stdenv.mkDerivation (finalAttrs: {
   buildInputs = [ openmp ];
 
   buildFlags = [ (lib.optional (openmp == null) "NO_OPENMP=1") ];
+
+  # clang builds require at least C99 or the build fails with:
+  # error: unknown type name 'inline'
+  env.NIX_CFLAGS_COMPILE = "-std=c99";
+
   installFlags = [ "PREFIX=$(out)" ];
 
   meta = with lib; {