The thrift compiler (since 0.13 at least) complains that
the csharp target is deprecated and recommend replacing it
with netstd. Since we don't use either, humor it.
I suspect that this warning caused some spurious rebuilds,
but have not proven it.
org::apache::cassandra (the generated namespace name) gets confused with
apache::cassandra (the thrift runtime library namespace), either due to
changes in gcc 7 or in thrift 0.10. Either way, the problem is fixed
by changing the generated namespace to plain cassandra.
The generic thrift headers bring in a #define (yuch) named VERSION, while the
Cassandra interface also defines a symbol with the same name.
Rename the symbol to avoid a compile conflict.
