[rubygems/rubygems] Do not use IO#flock on Solaris

`io.flock(File::LOCK_EX)` fails on Solaris when the io is opened as
read-only. Due to this issue, `make install` of the ruby package failed
on Solaris.

rubygems/rubygems@5905d17ab2

lib/rubygems.rb

@@ -788,7 +788,7 @@ def self.write_binary(path, data)
 
   def self.open_with_flock(path, flags, &block)
     File.open(path, flags) do |io|
-      unless java_platform?
+      if !java_platform? && !solaris_platform?
         begin
           io.flock(File::LOCK_EX)
         rescue Errno::ENOSYS, Errno::ENOTSUP
@@ -1015,6 +1015,13 @@ def self.java_platform?
     RUBY_PLATFORM == "java"
   end
 
+  ##
+  # Is this platform Solaris?
+
+  def self.solaris_platform?
+    RUBY_PLATFORM =~ /solaris/
+  end
+
   ##
   # Load +plugins+ as Ruby files