Make ThriftConfig.cmake relocatable

The current ThriftConfig.cmake isn't relocatable. It means that we can't move ThriftConfig.cmake installed by make install.

In general, users don't need to move ThriftConfig.cmake installed by make install but some packaging systems such as conda and MSYS2 needs to move. Because users of these packaging systems may use different prefix. For example, conda may be used with ~alice/my-conda and ~bob/conda as base directory.

CMake has a feature to make Config.cmake relocatable: https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html