...after 57949716 "Bump GCC baseline to 4.8.1"

Make this a fatal configuration error for now.  The check should be removed
completely after LO 6.0 branch-off.

Change-Id: I70cf65d6b0eb7158008f28449794c66c1b775916
Reviewed-on: https://gerrit.libreoffice.org/38869Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>