Disable -Weffc++

This throws up warnings (and therefore errors) when a struct is inside a
struct even if the inner-struct is initialized.  See the following bug:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55837

For some reason the CXX flags aren't working in the DDM4 in libdrizzle 5.1 so this is the first time I've spotted it