cmake: Compute non-CMake POD binary directory explicitly
cmake: Compute non-CMake POD binary directory explicitly

Compute `${proj}_BINARY_DIR` for non-CMake PODs that build in-source.
If a value is already set then use it instead.  This will allow projects
to specify in-source builds in a subdirectory of their actual source
tree.

billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake