wgrib2 library should be built without any extra features

[kgerheiser]

The library shouldn't have dependencies on Jasper, PNG, etc. It's the executable that really needs those features and having the library depend on it propagates those dependencies to builds that use the wgrib2 library.

I used object libraries to share object files between the executable and library, but building them separately for the library and executable may be necessary. Maybe with a default option to only build the executable.