fix broken Windows builds

First issue: pgroup is not available on Windows. new_pgroup should be
used instead. This broke in commit 790755c
(#763). Fixes #791.

Second issue: the mingw libssh2 package, which is required for building rugged,
is not installed out of the box on RubyInstaller with MSYS2.
This PR adds it to the dependency list so that installing the rugged gem
automatically pulls in the mingw libssh2 package as documented here:
https://github.com/oneclick/rubyinstaller2/wiki/For-gem-developers#-msys2-library-dependency
(related to PR #770).

Author: cfillion

ext/rugged/extconf.rb

@@ -30,7 +30,8 @@ class CMakeTimeout < StandardError
 
 def self.run_cmake(timeout, args)
   # Set to process group so we can kill it and its children
-  pid = Process.spawn("cmake #{args}", pgroup: true)
+  pgroup = Gem.win_platform? ? :new_pgroup : :pgroup
+  pid = Process.spawn("cmake #{args}", pgroup => true)
 
   Timeout.timeout(timeout) do
     Process.waitpid(pid)

rugged.gemspec

@@ -31,4 +31,5 @@ desc
   s.add_development_dependency "rake-compiler", ">= 0.9.0"
   s.add_development_dependency "pry"
   s.add_development_dependency "minitest", "~> 5.0"
+  s.metadata["msys2_mingw_dependencies"] = "libssh2"
 end