Summary:
I tried to build eden using GitHub version and ran into SELinux issues.
This patch fixed related issues, namely:
- `set(X Y)` sets X to string literal Y. Change it to `set(X ${Y})`.
- `SELINUX_INCLUDE_DIR` could be undefined. Check before use it.
- `./eden/fs/eden-config.h` and `./build/eden/fs/eden-config.h` both exist and
the former is used. Set include dirs so the latter gets used.
Reviewed By: bolinfest
Differential Revision: D9029272
fbshipit-source-id: 0c94bbe2f9e3fa90973802ddde16ad4d9ddfc0e8