#14 improve installation destination handling

[hasufell]

I recommend using the cmake module GNUInstallDirs and some logic like:

RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}"
LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}"
ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}"
FRAMEWORK DESTINATION "${MYGUI_FRAMEWORK_PATH}"

same for the pkgconfig destination which might even fail on some systems cause it does not respect libsuffix:
install(FILES ${MYGUI_BINARY_DIR}/pkgconfig/MYGUI${MYGUI_LIB_SUFFIX}.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)

On top of that, users can override those destinations if needed.