isl: move gdb script to gdb auto-load directory

This is where e.g. glib puts similar scripts; potentially gdb from
homebrew-versions needs to be configured to look here instead of in its
keg, but I'm not an expert.

Formula/isl.rb

@@ -14,5 +14,6 @@ class Isl < Formula
   def install
     system "./configure", "--disable-dependency-tracking", "--prefix=#{prefix}"
     system "make install"
+    (share/"gdb/auto-load").install Dir["#{lib}/*-gdb.py"]
   end
 end