Fix warning.h issue

vs2013 couldn't see warning.h in parent folder, even though it was
explicitly added to project. Renaming warning.h to warnmsg.h magically
solved the issue. Strangely enough both vs2013 and Windows SDK don't
have warning.h, so no idea why this name is so special.
Updage .gitignore

exclude temporary files from GNU Flex, such as "~1_flex_temp_GXMhMq"