build#install: install metafiles in buildpath

This came up in https://github.com/Homebrew/homebrew-emacs/pull/158#issuecomment-172031003;  `install_metafiles` is passed the current directory, which can be changed during installation by `Dir.chdir`.

There may be cases where the metafiles are in a subdirectory, but my
guess is those are rare and this is brittle and undocumented behavior
anyway.

Library/Homebrew/build.rb

@@ -133,7 +133,7 @@ class Build
         Tab.create(formula, ENV.compiler, stdlibs.first, formula.build, formula.source_modified_time).write
 
         # Find and link metafiles
-        formula.prefix.install_metafiles Pathname.pwd
+        formula.prefix.install_metafiles formula.buildpath
         formula.prefix.install_metafiles formula.libexec if formula.libexec.exist?
       end
     end