In #26252 support was added to have prettier paths printed out on failure by not
passing the full path to the source file to the compiler, but instead just a
small relative path. To preserve this relative path across configurations, the
`SREL` variable was used for reconfiguring, but if `SREL` is empty then it will
attempt to run the command `configure` which is distinct from running
`./configure` (e.g. doesn't run the local script).
This commit modifies the `SREL` value to re-run the configure script by setting
it to `./` in the case where `SREL` is empty.
9ad0063a17