feat(build): add cache var to disable examples

[gf3]

Add OBX_DISABLE_EXAMPLES with a default value of OFF to control including the examples directory in the default build.

Example

You can disable including/building the examples by setting the cache variable OBX_DISABLE_EXAMPLES to ON:

-DOBX_DISABLE_EXAMPLES=ON

E.g. in KDevelop project CMake configuration:

Closes #33