In the case, where IDL enums start with a character, that is an invalid
start character for C++ identifiers (e.g. a number), the C++ generaion
for enums fails. An example would be "2d" see #3788
Previously the IDL Parser Complained, that a type with the name ''
(an empty string) couldn't be found. It wasn't that easy to see the
mistake, as the not named type is printed without '' around it, so the
message seemed to miss a type. This now catches this specify error
earlier and reports it cleanly to the user. An example of this
occurring would be ''typedef A (B or //FIXME: C )
This allows us to specify multiple base paths to look for imported IDL
files in. This will allow us to import IDL files from sources and from
the Build directory (i.e. for generated IDL files).
