Detect if rubygems are running under rpmbuild.

This allows to install binaryi extensions on suitable place and clears
the .spec files a bit.

operating_system.rb

@@ -11,6 +11,15 @@ module Gem
     end
     private :previous_but_one_dir_to
 
+    ##
+    # Tries to detect, if arguments and environment variables suggest that
+    # 'gem install' is executed from rpmbuild.
+
+    def rpmbuild?
+      (ARGV.include?('--install-dir') || ARGV.include?('-i')) && ENV['RPM_PACKAGE_NAME']
+    end
+    private :rpmbuild?
+
     ##
     # Default gems locations allowed on FHS system (/usr, /usr/share).
     # The locations are derived from directories specified during build
@@ -78,8 +87,16 @@ module Gem
     end
 
     def default_ext_dir_for base_dir
-      dirs = Gem.default_dirs.detect {|location, paths| paths[:gem_dir] == base_dir}
-      dirs && File.join(dirs.last[:ext_dir], RbConfig::CONFIG['RUBY_INSTALL_NAME'])
+      dir = if rpmbuild?
+        build_dir = base_dir.chomp Gem.default_dirs[:system][:gem_dir]
+        if build_dir != base_dir
+          File.join build_dir, Gem.default_dirs[:system][:ext_dir]
+        end
+      else
+        dirs = Gem.default_dirs.detect {|location, paths| paths[:gem_dir] == base_dir}
+        dirs && dirs.last[:ext_dir]
+      end
+      dir && File.join(dir, RbConfig::CONFIG['RUBY_INSTALL_NAME'])
     end
 
     # This method should be available since RubyGems 2.2 until RubyGems 3.0.