This allows user to specify any name in `--with-so-name` that might
cause a name clash with LIBRUBY_ALIASES on the platform.
Without this, for example, configuring with `--with-soname=ruby
--enable-shared` on macOS would end up running `ln -sf libruby.dylib
libruby.dylib` only to fail with the following error in installation:
```
make[2]: stat: libruby.dylib: Too many levels of symbolic links
```
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63544 b2dd03c8-39d4-4d8f-98ff-823fe69b080e