re: issue https://github.com/Unidata/netcdf-c/issues/1323
and PR https://github.com/Unidata/netcdf-c/pull/1326
Replace the uses of the __FILE__ macro in ncgen with the
constant string "ncgen" in order to keep external package
builders (like Debian) happy.
Note that __FILE__ will still appear in ncgen generated C code
files, but this should be ok since it is not part of the distro
per-se.