[ruby/private-ruby-2.1] Detect if rubygems are running under rpmbuild.

[Vít Ondruch]

commit 19ff4992f3753a8a614abf5088635b6fbd2464e0
Author: Vít Ondruch <vondruch at redhat.com>
Date:   Fri Jan 10 14:53:57 2014 +0100

    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 |   21 +++++++++++++++++++--
 1 files changed, 19 insertions(+), 2 deletions(-)
---
diff --git a/operating_system.rb b/operating_system.rb
index fba8117..079564f 100644
--- a/operating_system.rb
+++ b/operating_system.rb
@@ -12,6 +12,15 @@ module Gem
     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
     # configuration.
@@ -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.