What is the difference between these two sentence:( <> and " ")?
#include <rtl.h>
#include "rtl.h"
Yes, it is definitely better to have 3rd-party libraries hard-linked to projects, instead of having them installed system-wide and after an update of the library find out that 3 out of 12 projects suddenly doesn't build.
I normally install extra libraries in directories that specifies the exact library version and have the different project specify exactly what library version to use. That allows multiple versions to be available at the same time and existing projects can be rebuilt without any changes.
Another danger when having extra header files in the <> namespace is the potential namespace collision. It isn't so fun when the order of the path controls which header files that gets used.