Fixed the location of the shared library

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.

Author: nobu

library/rbconfig/rbconfig_spec.rb

@@ -39,7 +39,9 @@
 
   guard -> {RbConfig::TOPDIR} do
     it "libdir/LIBRUBY_SO is the path to libruby and it exists if and only if ENABLE_SHARED" do
-      libdir = RbConfig::CONFIG[RbConfig::CONFIG['libdirname'] || 'libdir']
+      libdirname = RbConfig::CONFIG['LIBPATHENV'] == 'PATH' ? 'bindir' :
+                     RbConfig::CONFIG['libdirname']
+      libdir = RbConfig::CONFIG[libdirname]
       libruby_so = "#{libdir}/#{RbConfig::CONFIG['LIBRUBY_SO']}"
       case RbConfig::CONFIG['ENABLE_SHARED']
       when 'yes'

optional/capi/spec_helper.rb

@@ -32,11 +32,9 @@ def compile_extension(name)
   ruby_header = "#{RbConfig::CONFIG['rubyhdrdir']}/ruby.h"
 
   if RbConfig::CONFIG["ENABLE_SHARED"] == "yes"
-    if PlatformGuard.windows?
-      libruby_so = "#{RbConfig::CONFIG['bindir']}/#{RbConfig::CONFIG['LIBRUBY_SO']}"
-    else
-      libruby_so = "#{RbConfig::CONFIG['libdir']}/#{RbConfig::CONFIG['LIBRUBY_SO']}"
-    end
+    libdirname = RbConfig::CONFIG['LIBPATHENV'] == 'PATH' ? 'bindir' :
+                   RbConfig::CONFIG['libdirname'] # defined since 2.1
+    libruby_so = "#{RbConfig::CONFIG[libdirname]}/#{RbConfig::CONFIG['LIBRUBY_SO']}"
   end
 
   begin