Resulting CMake Config is not relocatable

### Expected behavior

The generated [CMake Config](https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#config-file-packages) under `/usr/lib/cmake/rocksdb` is relocatable and does not contain any fixed paths to the build location.

### Actual behavior

The generated [CMake Config](https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html?highlight=config#config-file-packages) under `/usr/lib/cmake/rocksdb` is **not** relocatable and does contain fixed paths to the build location.

### Steps to reproduce the behavior

Build rocksdb using CMake and inspect the resulting files in `/usr/lib/cmake/rocksdb`. It contains somthing like the following:

```
set_target_properties(RocksDB::rocksdb PROPERTIES
INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
INTERFACE_LINK_LIBRARIES "/builds/recipe-sysroot/usr/lib/libz.so;/builds/recipe-sysroot/usr/lib/libbz2.so;/builds/recipe-sysroot/usr/lib/liblz4.so;-lpthread"
)
```
With absolute paths to the build directory `/builds`.

### Analysis

This is a well known issue with CMake and it is even [documented in the CMake Manual](https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#creating-relocatable-packages).

In a nutshell the issue is that the CMake concepts used RocksDB are old and from the 2.8 era. Modern CMake recomends to use [Imported Targets](https://cmake.org/cmake/help/latest/command/add_library.html#imported-libraries) which does not suffer from the same issue. But it requires a higher minumum CMake version (3.7+) to work properly.

### Proposed Solution

- Increase the minumum required CMake version to 3.7
- Rework the finding of external libraries to make use of imported targets