sdcc: add reminder to remove .PHONY patch with next release

pkgs/by-name/sd/sdcc/package.nix

@@ -68,8 +68,15 @@ stdenv.mkDerivation (finalAttrs: {
   # filesystem), the INSTALL file keeps the `install` target in the ucsim
   # directory from running. Nothing else creates the `man` output, causing the
   # entire build to fail.
+  #
+  # TODO: remove this when updating to the next release - it's been fixed in
+  # upstream sdcc r15384 <https://sourceforge.net/p/sdcc/code/15384/>.
 
   postPatch = ''
+    if grep -q '\.PHONY:.*install' sim/ucsim/Makefile.in; then
+      echo 'Upstream has added `.PHONY: install` rule; must remove `postPatch` from the Nix file.' >&2
+      exit 1
+    fi
     echo '.PHONY: install' >> sim/ucsim/Makefile.in
   '';