ndk ver. 21
The ndk had some breaking changes in version 21 or 20, not sure which one. So the SDK manager does also provide an older NDK version which you can use for now, which works for SFML. It seems like they changed the paths findPackage looks for and therefore older cmake code breaks.
To fix this you could remove the findPackage call for android and just link gles directly, since it is guaranteed to be provided by the NDK anyway.
This problem is caused by the lack of "GLES" directory in /usr/include.
This path should not matter since the android toolchain should not look at your host system's header files which may/will be different to the one the actual GLES shared library was created from.