Factor out script to build index.hdr

As part of factoring out the documentation building parts of the fish
build, add a new file build_index_hdr.sh that builds the index.hdr
file. Invoke it from both the Makefile and CMake build.

build_tools/build_index_hdr.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+TOC_TXT=$1
+env awk "{if (\$0 ~ /@toc@/){ system(\"cat ${TOC_TXT}\");} else{ print \$0;}}"
+