use png for favicon, generate thumbnails in background on build, and return exit status from build

build.sh

@@ -45,8 +45,12 @@ if [ "$1" = "-t" ]; then
   shift && ./tw.sh $@
 fi
 ./main
-./thumbnailer.sh build
-
+exitstatus=$?
+if [ "${exitstatus}" = "0" ]; then
+  ./thumbnailer.sh build &
+fi
 
 # Cleanup
 rm -f ./main
+
+exit ${exitstatus}

main.c

@@ -535,7 +535,7 @@ FILE* build(FILE* f, Lexicon* l, char* name, char* srcpath) {
   }
   fprintf(
     f,
-    "<link rel='shortcut icon' href='/favicon.ico'>\n"
+    "<link rel='shortcut icon' href='/favicon.png'>\n"
     "<title>" NAME " &mdash; %s</title>\n",
     ismetanav(name) ? "home" : name);
   fputs("</head>\n", f);