binutils: fix libiberty --enable-install-libiberty flag

This patch fixes the compatibility osx applications that need to work libiberty but without third-party gcc version above `binutils 2.23` was the damaged file is configuration, incorrect condition.

binutils fix `--enable-install-libiberty` flag fixed the `--disable-install-libiberty` behavior, but it also added a bug where the enable path never works because the initial clear of target_header_dir wasn't deleted.  So we end up initializing properly at the top only to reset it at the end all the time.  http://gcc.gnu.org/ml/gcc-patches/2014-01/msg00213.html
[`20_all_libiberty-install.patch`](http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo/src/patchsets/binutils/2.24/20_all_libiberty-install.patch)

```ruby
#{Formula['binutils'].lib}/x86_64/libiberty.a
```