compress-man-pages: optimize, use multiple cores (#406922)

pkgs/build-support/setup-hooks/compress-man-pages.sh

@@ -9,15 +9,9 @@ compressManPages() {
     echo "gzipping man pages under $dir/share/man/"
 
     # Compress all uncompressed manpages.  Don't follow symlinks, etc.
+    # gzip -f is needed to not error out on hard links.
     find "$dir"/share/man/ -type f -a '!' -regex '.*\.\(bz2\|gz\|xz\)$' -print0 \
-        | while IFS= read -r -d $'\0' f
-    do
-        if gzip -c -n "$f" > "$f".gz; then
-            rm "$f"
-        else
-            rm "$f".gz
-        fi
-    done
+        | xargs -0 -n1 -P "$NIX_BUILD_CORES" gzip -f
 
     # Point symlinks to compressed manpages.
     find "$dir"/share/man/ -type l -a '!' -regex '.*\.\(bz2\|gz\|xz\)$' -print0 \