[Cuse/Fix] Add ENABLE_CUSE macro instead of variable

[Cuse/Fix] Add ENABLE_CUSE macro instead of variable

This patch explicitly adds ENABLE_CUSE macro rather than
using an internal variable.

Note that in case of .NET app running on the Tizen emulator,
cuse context's pid is not the calling process. So, the pid
info is shared via a file. But, this may not have any sync issue
because an emulator may not support concurrent executions.

Signed-off-by: Dongju Chae <dongju.chae@samsung.com>