build system: protect digest calls from re-entry

The code for Ruby's Digest module seems to be racy
wrt. multi-threading. See e.g.
https://github.com/aws/aws-sdk-ruby/issues/525 for details.

Rakefile

@@ -47,6 +47,7 @@ $build_system_modules = {}
 $have_gtest           = (c(:GTEST_TYPE) == "system") || (c(:GTEST_TYPE) == "internal")
 $gtest_apps           = []
 
+require_relative "rake.d/digest"
 require_relative "rake.d/helpers"
 require_relative "rake.d/target"
 require_relative "rake.d/application"

rake.d/digest.rb

@@ -0,0 +1,7 @@
+require "digest"
+
+$digest_mutex = Mutex.new
+
+def sha1_hexdigest data
+  $digest_mutex.synchronize { Digest::SHA1.hexdigest(data) }
+end

rake.d/format_string_verifier.rb

@@ -1,7 +1,5 @@
 #!/usr/bin/env ruby
 
-require "digest"
-
 class FormatStringVerifier
   def verify file_name
     language  = file_name.gsub(%r{.*/|\.po$}, '')
@@ -35,7 +33,7 @@ class FormatStringVerifier
           reject(&:nil?).
           each do |msgid|
             format_types.each do |type|
-              sha1 = Digest::SHA1.hexdigest "#{type}:#{msgid}:#{msgstr}"
+              sha1 = sha1_hexdigest "#{type}:#{msgid}:#{msgstr}"
 
               next if to_ignore.key?(language) && to_ignore[language].include?(sha1)