Added proper install configuration for cmake

[artudi54]

I have made some CMake cleanup and added proper install configuration. Now the library can be found with "find_package(E57Format)" or used as module with "add_subdirectory(....)" without the need of any additional configuration.
Library target automatically links the xerces-c library to itself, and specifies proper public/private include directories.
Install command let's you now find the package from any other cmake project with xerces-c dependency already solved.

I have tested this functionalities on both linux and windows. If you find have any problems with this update or something is not working right please let me know.

[asmaloney]

Thanks @artudi54.

Do any of these changes require cmake > 3.1?

[artudi54]

According to the documentation, install, target_compile_definitions, target_include_directories, target_link_libraries and CMakeFindDependencyMacro are all available in CMake 3.1. Every parameter, option or keyword which I used can be found in the docs, so there is no need to increase the minimal version.

[asmaloney]

Thank you @artudi54!