Merged r18126 from trunk to 4.0-stable (#31125).

git-svn-id: http://svn.redmine.org/redmine/branches/4.0-stable@18127 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine/thumbnail.rb

@@ -49,7 +49,12 @@ module Redmine
 
     def self.convert_available?
       return @convert_available if defined?(@convert_available)
-      @convert_available = system("#{shell_quote CONVERT_BIN} -version") rescue false
+      begin
+        `#{shell_quote CONVERT_BIN} -version`
+        @convert_available = $?.success?
+      rescue
+        @convert_available = false
+      end
       logger.warn("Imagemagick's convert binary (#{CONVERT_BIN}) not available") unless @convert_available
       @convert_available
     end