capnp_generate_cpp() checks that input schema files exist at configure-time, and reports a fatal error if they don't exist. However, the check prepended the value of CAPNPC_SRC_PREFIX to input file paths, which is the wrong thing to do: the input file paths should be checked as-is if they are absolute paths, and checked relative to the current source directory, NOT the value of capnp's src-prefix flag, if they are relative paths, in order to match the capnp tool's behavior.

It turns out that it's easiest to just unconditionally convert the input file paths to absolute paths, then check the absolute path. The reason is that we can't even pass relative paths to the capnp command, anyway: capnp interprets relative path inputs relative to its working directory, which defaults to the build dir. For consistency with other CMake commands (add_library, add_executable, etc.), it makes most sense if relative file path inputs to capnp_generate_cpp() are interpreted relative to the current source directory. This means that relative path inputs need to be converted to absolute paths before being fed to capnp, which was done right after the faulty existence check.

This commit fixes the issue by modifying the existence check to check the path only after it's been converted to absolute form.

Closes #586.