djbdns: add clt workaround

Rewrote most of the formula to avoid echoing stuff into files and just
rewrite them ourselves, since calling the system on that sort of thing is
pretty redundant & slow.

Closes Homebrew/homebrew#42478.

Formula/djbdns.rb

@@ -5,15 +5,29 @@ class Djbdns < Formula
   sha256 "3ccd826a02f3cde39be088e1fc6aed9fd57756b8f970de5dc99fcd2d92536b48"
 
   depends_on "daemontools"
+  depends_on "ucspi-tcp"
 
   def install
     inreplace "hier.c", 'c("/"', "c(auto_home"
     inreplace "dnscache-conf.c", "/etc/dnsroots", "#{etc}/dnsroots"
-    system "echo gcc -O2 -include /usr/include/errno.h > conf-cc"
-    system "echo #{prefix} > conf-home"
-    system "echo gcc > conf-ld"
+
+    # Write these variables ourselves.
+    rm %w[conf-home conf-ld conf-cc]
+    (buildpath/"conf-home").write prefix
+    (buildpath/"conf-ld").write "gcc"
+
+    if MacOS::CLT.installed?
+      (buildpath/"conf-cc").write "gcc -O2 -include /usr/include/errno.h"
+    else
+      (buildpath/"conf-cc").write "gcc -O2 -include #{MacOS.sdk_path}/usr/include/errno.h"
+    end
+
     bin.mkpath
-    (prefix+"etc").mkpath
-    system "make setup check"
+    (prefix/"etc").mkpath # Otherwise "file does not exist"
+    system "make", "setup", "check"
+  end
+
+  test do
+    assert_match /localhost/, shell_output("#{bin}/dnsname 127.0.0.1")
   end
 end