support gccEmacs

If built with gccEmacs, in `straight--native-compile-package` step tries
to access $HOME because `comp-eln-load-path` contains
"~/.emacs.d/eln-cache/". It errors on this as it is set to a readonly canary value of
`/homeless-shellter` in the nix build sandbox.

The workaround is twofold:
 * Ignore `kill-emacs` from optimization. It would result in eln file
   being written before doom sets up the load path outside of HOME.
 * The above solution doesn't help in the actual build phase because the
   worker processes compiling packages does not inherit this value but
   it is set in them at a later time. This causes the build to fail
   before `comp-eln-load-path` would be successfully overridden. Having
   a writable $HOME does not seem to produce any build output there
   hence the override and the check in `postInstall` to verify that the
   directory is indeed empty.

The runtime value of `comp-eln-load-path` also had to be changed because
it is by default is located under the volatile `doom-cache-dir` which is
not a part of the derivation so the build time generated `.eln` files
couldn't be loaded from that.

References: #57

advice.el

@@ -7,6 +7,12 @@
 ;;; have a properly configured user home and environment.
 (setq package-check-signature nil)
 
+;;; For gccEmacs compatibility
+(with-eval-after-load "comp"
+  ;; The advice for 'kill-emacs would result in eln files being written before
+  ;; doom would set up proper load paths
+  (add-to-list 'comp-never-optimize-functions 'kill-emacs))
+
 (defun nix-straight-inhibit-kill-emacs (arg)
   (message "[nix-doom-emacs] Inhibiting (kill-emacs)"))
 

default.nix

@@ -94,6 +94,7 @@ let
   fmt = {
     reset=''\\033[0m'';
     bold=''\\033[1m'';
+    red=''\\033[31m'';
     green=''\\033[32m'';
   };
 
@@ -146,6 +147,22 @@ let
       preInstall = ''
           export DOOMDIR=${doomPrivateDir}
           export DOOMLOCALDIR=$out/
+
+          # Create a bogus $HOME directory because gccEmacs is known to require
+          # an existing home directory because the async worker process don't
+          # fully respect the value of 'comp-eln-load-path'.
+          export HOME=$(mktemp -d)
+      '';
+      postInstall = ''
+        # If gccEmacs or anything would write in $HOME, fail the build.
+        if [[ -z "$(find $HOME -maxdepth 0 -empty)" ]]; then
+          printf "${fmt.red}${fmt.bold}ERROR:${fmt.reset} "
+          printf "${fmt.red}doom-emacs build resulted in files being written in "'$HOME'" of the build sandbox.\n"
+          printf "Contents of "'$HOME'":\n"
+          find $HOME
+          printf ${fmt.reset}
+          exit 33
+        fi
       '';
     });
 

nix-integration.patch

@@ -26,3 +26,12 @@ index c8cfb0495..d8ed107bd 100644
    "Directory for volatile local storage.
  
  Use this for files that change often, like cache files. Must end with a slash.")
+@@ -276,7 +276,7 @@ config.el instead."
+ ;; Don't store eln files in ~/.emacs.d/eln-cache (they are likely to be purged
+ ;; when upgrading Doom).
+ (when (boundp 'comp-eln-load-path)
+-  (add-to-list 'comp-eln-load-path (concat doom-cache-dir "eln/")))
++  (add-to-list 'comp-eln-load-path (concat doom-local-dir "cache/eln/")))
+ 
+ (after! comp
+   ;; HACK `comp-eln-load-path' isn't fully respected yet, because native