Allow building phobos externally

This allows building phobos and druntime with an existing gdc
compiler. This is meant to simplify porting to other
architectures.

Implementation details:
* Detect if we're compiling out of gcc tree in configure.in
* Set multi_basedir = srcdir if compiling out of tree, so
  config-ml.in can be found
* GCC_NO_EXECUTABLES isn't defined outside of gcc
* Do a proper check for AR to get the correct ar for cross compilers
* Remove no longer needed libiberty dependency
* Fix using system zlib
* Force using system zlib for out of tree builds
Remove unused variables from phobos-ver-syms