Stop blowing away all symlinks in clean.sh

make.sh should not have the default behavor of destroying work-in-progress
involving symlinks. The "solution" of creating a file of precious names was
not very discoverable and I only stumbled upon it after I thought I saw
symlinks disappear several times in a row, before I realized that I was not
imagining it.

clean.sh

@@ -4,16 +4,10 @@ set -e
 # Remove everything in directories which are only used for output.
 # In most cases, we can remove the directories, too.
 #
-# (We don't remove all the directories themselves for a stupid technical
-# reason: "gmake clean" in the src/runtime directory gets unhappy if the
-# output/ directory doesn't exist, because it tries to build Depends
-# before it cleans itself, and src/c-runtime/sbcl.h is a symlink into
-# the output/ directory, and it gets the gcc dependency processing gets
-# all confused trying to figure out a header file which is a symlink
-# into a directory which doesn't exist. We'd like to be able to run
-# this script (including "gmake clean" in the src/runtime directory)
-# several times in a row without failure.. so we leave the output/
-# directory in place.)
+# Preserving the skeletons of some directories might or might not not be relevant.
+# The supposed explanation definitely no longer holds - there is no "src/c-runtime/sbcl.h"
+# as a symlink into "output/". The sbcl.h file is a plain file, and it's been that way
+# for quite some time.
 rm -rf obj/* output/* src/runtime/genesis/ src/runtime/sbcl.mk
 
 # Ensure that we know GNUMAKE.
@@ -36,8 +30,6 @@ done
 
 # Within all directories, remove things which don't look like source
 # files. Some explanations:
-#   (symlinks)
-#     are never in the sources, so must've been created
 #   sbcl
 #     the runtime environment, created by compiling C code
 #   sbcl.h
@@ -78,7 +70,7 @@ find . \( \
         -name .git -o \
         -name .svn \) -type d -prune -o \
        \( \
-        -type l -o \
+        \( -type l -path ./src/runtime/\* \) -o \
         -name '*~' -o \
         -name '#*#' -o \
         -name '.#*' -o \
@@ -117,15 +109,4 @@ find . \( \
         -name 'depend' -o \
         -name 'TAGS' -o \
         -name 'tags' -o \
-        -name 'local-target-features.lisp-expr' \) -print | \
-    if test -f .cleanignore; then
-        # Because this file deletes all symlinks, it prevents building
-        # in a tree of symlinks. Here's a low-tech workaround: have
-        # whatever tool creates your tree of symlinks enumerate
-        # relative paths to each one in a file called .cleanignore,
-        # and this script won't delete them. This .cleanignore file
-        # doesn't support any wildcards or comments.
-        awk 'BEGIN{while(getline <".cleanignore"!=0){ign["./" $0]=1}} ign[$0]!=1';
-    else
-        cat;
-    fi | xargs rm -fr
+        -name 'local-target-features.lisp-expr' \) -print | xargs rm -fr