Revert "No need to explicitly set builddir as it’s already among our variables"

There may be situations in which the user already have (generated) ninja build files which do not match our assertions.

This reverts commit b42cbb1f7e.

bin/gen_build

@@ -925,6 +925,7 @@ if __FILE__ == $PROGRAM_NAME
 
     bootstrap = Buildfile.new(nil, builddir, variables)
     bootstrap.assign('ninja_required_version', '1.5')
+    bootstrap.assign('builddir', builddir)
 
     args = variables.reject { |key, value| config.keys.include?(key) }
     args = args.map { |key, value| "-d'#{key}=#{value =~ /\$/ ? value.gsub(/\$/, '$$') :  "$#{key}"}'" }.join(' ')