When generating code for multiple ASN.1 syntaxes that have clashing
names, we need to add a prefix in order to prevent clashes in the global
C symbol namespace.  Using the ASN1C_PREFIX environment variable and
this patch serves as a work-around to that.  All non-basic type names
as well as references to that type and source code + header file names
will be pre-fixed accordingly.

This is a re-implementation of this feature due to osmocom's
implementation version can not work on newest asn1c repository.