CMake enhancements

* Add magic_enum as an interface library
* Enable tests and examples only if is top project
* Use more modern approach to include directories
* Add install target

install.cmake

@@ -0,0 +1,25 @@
+include(CMakePackageConfigHelpers)
+write_basic_package_version_file(
+        ${PROJECT_NAME}ConfigVersion.cmake
+        VERSION ${PROJECT_VERSION}
+        COMPATIBILITY AnyNewerVersion
+)
+
+install(TARGETS ${PROJECT_NAME}
+        EXPORT ${PROJECT_NAME}Targets
+        PUBLIC_HEADER DESTINATION include
+        INCLUDES DESTINATION include
+        )
+
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake
+        DESTINATION lib/cmake/${PROJECT_NAME}
+        )
+
+install(EXPORT ${PROJECT_NAME}Targets
+        NAMESPACE ${PROJECT_NAME}::
+        DESTINATION lib/cmake/${PROJECT_NAME}
+        )
+
+install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/include
+        DESTINATION include
+        )