On platform where searchs shared libraries by `PATH` environment
variable (i.e., Windows), the shared library is installed in
`bindir`. On other platforms, the library directory is directed
by `libdirname` indirectly.
https://github.com/ruby/ruby/runs/541455267
```
1)
RbConfig::CONFIG libdir/LIBRUBY_SO is the path to libruby and it exists if and only if ENABLE_SHARED FAILED
Expected File.exist? "d:/Ruby26-x64/lib/x64-msvcrt-ruby260.dll"
to be truthy but was false
D:/a/ruby/ruby/src/spec/ruby/library/rbconfig/rbconfig_spec.rb:46:in `block (3 levels) in <top (required)>'
D:/a/ruby/ruby/src/spec/ruby/library/rbconfig/rbconfig_spec.rb:4:in `<top (required)>'
```
