Summary:
pkg-config files are quite useful for communicating to users of a
library how to compile against them. This commit generates and installs
a pkg-config file that can be used for both static and dynamic builds
against the RocksDB library. This should make life easier for developers
of client programs, language bindings, etc.

Example usage:

```
g++ `pkg-config --cflags rocksdb` -o simple_example simple_example.cc `pkg-config --libs rocksdb`

g++ `pkg-config --cflags --static rocksdb` -static \
   -o simple_example simple_example.cc `pkg-config --libs --static rocksdb`
```

The commit also adds the generated file to .gitignore, to the uninstall
target, and to clean.

No additional dependencies are added to RocksDB itself, and this does
not make RocksDB use pkg-config as part of its build process.

Resolves https://github.com/facebook/rocksdb/issues/4452

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7244

Reviewed By: siying

Differential Revision: D23146153

Pulled By: ajkr

fbshipit-source-id: 3045aa650d68bd5ac42d40ed709570e9584ef004