Support building the libraries using a vendored copy of the C++ library

[jorisdral]

I've been looking into the options that the Rust bindings of the C++ library provide. See https://github.com/randombit/botan-rs. They provide three options:

1. includes: find the C++ by probing for the library using the usual search paths
2. pkg-config: find the C++ library using pkg-config
3. vendored: build a copy of the C++ library that is distributed with the package

Right now, the Haskell libraries support options 1 and 2. Option 1 is supported by disabling the pkg-config Cabal flag for the botan-bindings package, and providing the path to the C++ library using --extra-lib-dirs, and providing the path to the corresponding header files using --extra-include-dirs. Option 2 is supported by enabling the pkg-config flag, and then pkg-config is invoked by Cabal automatically. Either option still requires installing the library by hand, either via a package manager or by building the C++ library from source.

Option 3 is not yet supported. I think we should probably support option 3 as well, if possible.

Originally posted by @jorisdral in #41