CMake: install relocatable shared libs for macOS

[SpaceIm]

Remove hardcoded CMAKE_INSTALL_NAME_DIR, so that it's @rpath by default (default CMake behavior when cmake_minimum_required() is at least 3.0). set(CMAKE_MACOS_RPATH TRUE) is useless, it's the default value for CMake if cmake_minimum_required() is at least 3.0 as well, no need to hardcode this value.

It reverts #976 (hardcoding CMAKE_INSTALL_NAME_DIR is bad practice).

If someone wants to change this behavior for a specific reason, CMAKE_INSTALL_NAME_DIR can be injected externally, no need to patch build files.

[eustas]

Hello. I'll prepare a middle-ground PR:

• remove set(CMAKE_MACOS_RPATH
• remove set(CMAKE_INSTALL_NAME_DIR
• specify default INSTALL_NAME if CMAKE_INSTALL_NAME_DIR is not defined

[SpaceIm]

specify default INSTALL_NAME if CMAKE_INSTALL_NAME_DIR is not defined

Why?

[eustas]

Because that is what embedders of library do. Why not make their life easier?

[SpaceIm]

It makes life of package managers harder, because it changes default CMake behavior.