manually specify existing proj installation via env variables

For most people, pkg-config is a great option. But for when it's not, I think conventions exist for manually specifying environment variables.

Something like:

```
PROJ_SYS_PROJ_INCLUDE_DIR=/path/to/my/proj/include PROJ_SYS_LIBRARY_PATH=/path/to/my/proj/lib cargo build
```

We don't currently support this, but we should.

Start by locating the build entry point that currently relies on pkg-config for PROJ discovery. Test a build against an existing PROJ installation using PROJ_SYS_PROJ_INCLUDE_DIR and PROJ_SYS_LIBRARY_PATH, and consider the work done when those variables select the requested headers and library successfully.

Written by the indexing model from the issue text.