CMake: make winflexbison easier to use as a subdirectory

[ligfx]

If it's used as a subdirectory:

• doesn't change output directories
• doesn't set up install rules or CPack
• doesn't add USE_STATIC_RUNTIME global option

Also renamed common_lib to winflexbison_common

[lexxmark]

Thank you for pull requests, I hope they don't break any conan stuff. @Croydon could you please check if all changes from @ligfx are good for conan?

[Croydon]

I will have a look, but I'm quite busy right now.

If I should forget about it, please ping me again in several days

[lexxmark]

@Croydon pinging you to take a look at these pull requests