Export include directory when using Easylogging++ as a static library (Fix for #621)

[MaxSavenkov]

This makes the Easylogging++ more usable as a static library: now, the user can just add it as a sub-directory to his CMakeLists.txt and then user target_link_libraries( target_name easyloggingpp ) to both link the library to his project and add an include directory.

Also, I removed useless include_directories command which used a directory which (no longer?) exists.

The change only affects build system, and only for the case where build_static_lib flag is ON. It, however, requires a newer version of CMake (at least 2.8.11). I tested it with CMake 3.11, and it worked.