Pass internal gdb flags to --configuration invocations

Pass internal gdb flags to --configuration invocations

The test suite uses the --configuration flag to feature-test gdb.
However, when I added this, I neglected to pass the internal gdbflags
to this, causing an error, which then caused failures in the test
suite (which would not be seen if you'd ever run "make install").

This patch fixes the bug.  Tested by removing my install tree first,
to verify that I could reproduce the failure.