This makes it much easier to find a particular thing in config.log.

We have to use the ${BASH_LINENO[*]} syntax which is a syntax error in
other shells, so test what shell we are running and use eval.

The extra output is only printed if configure is run with bash.  On
systems where /bin/sh is not bash, it is necessary to say bash
./configure to get the extra debug info in the log.
Suggested-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NIan Jackson <Ian.Jackson@eu.citrix.com>
CC: Kent R. Spillner <kspillner@acm.org>
CC: Janosch Frank <frankja@linux.vnet.ibm.com>
CC: Thomas Huth <thuth@redhat.com>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: NThomas Huth <thuth@redhat.com>