Allow combination of _Thread_local with extern or static

[dspinellis]

C99 rules prevented the specification of more than one storage class specifier for a given object. Now _Thread_local can be combined with extern or static.

[dspinellis]

The proper way to do this is to split e_storage_class into e_storage_duration and e_linkage defining appropriate merge functions. c_enum must then be handled separately from e_storage_class.