Fix issues reported by rpmlint and fedora-review
Add 'dl' and gobject-2.0 dependencies for the main library
Use CFLAGS from environment for building

This e.g. adds the '-g' compile option when building RPMs.

Also let the environment to specify the compiler.
Fix issues reported by -Werror=format-security
Add a metapackage for installing all plugins

It's more convenient than listing them manually.