Allow setting a custom dir to pull the clang includes from

[expenses]

${CLANG_INSTALL_PREFIX}/lib/clang/${LLVM_VERSION_MAJOR}/include is invalid on systems that store the libs for clang seperately from wherever CLANG_INSTALL_PREFIX is.

E.g. on Nix/NixOS it points to /nix/store/0y99g3631mcl08ym46a5803g0hr4rc8j-clang-16.0.6 which looks like:

/nix/store/0y99g3631mcl08ym46a5803g0hr4rc8j-clang-16.0.6/
├── bin
...
│   ├── clang -> clang-16
│   ├── clang-16
...
└── share
    ├── clang
...

but also /nix/store/l92dv1igdpgkb1mxjv9rabvk795k45ih-clang-16.0.6-lib which looks like:

/nix/store/l92dv1igdpgkb1mxjv9rabvk795k45ih-clang-16.0.6-lib
├── lib
│   ├── clang
│   │   └── 16
│   │       └── include
│   │           ├── __clang_cuda_builtin_vars.h
...
│   ├── libclang-cpp.so -> libclang-cpp.so.16
│   ├── libclang-cpp.so.16
...

Using a define for this directory lets me overwrite the value like so:
https://github.com/expenses/babble-nix/blob/b5ecce0864ccdad936adadd6514faf26fe8e93f8/package.nix#L14-L16

I'm happy to bikeshed on the naming of this variable.