cp over CMakeLists.txt if cmake builds fail

The self help text does not mention the log, but we can add this later, and for now at least the file is salvaged.

Closes Homebrew/homebrew#5940.

Library/Homebrew/formula.rb

@@ -259,11 +259,14 @@ class Formula
         yield self
       rescue Interrupt, RuntimeError, SystemCallError => e
         unless ARGV.debug?
+          logs = File.expand_path '~/Library/Logs/Homebrew/'
           if File.exist? 'config.log'
-            logs = File.expand_path '~/Library/Logs/Homebrew/'
             mkdir_p logs
-            cp 'config.log', logs
+            mv 'config.log', logs
           end
+          if File.exist? 'CMakeLists.txt'
+            mkdir_p logs
+            mv 'CMakeLists.txt', logs
           raise
         end
         onoe e.inspect