`WK_CHECKOUT_PATH` defines location of webkit checkout on the
file system. All browser-related scripts, like `prepare_checkout.sh` and
`export.sh` respect this environment variable on all platforms.
Second try at #556. Uses absolute paths for the environment variables so that WPEDependencies builds properly. Switches from `WebKitBuildWPE` to `WebKitBuild/WPE` to avoid the need to change the .gitignore.
My computer takes a long time to build, but it appears to be working.