configure: do_compiler: Dump some extra info under bash

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>

configure

@@ -60,6 +60,11 @@ do_compiler() {
     # is compiler binary to execute.
     local compiler="$1"
     shift
+    if test -n "$BASH_VERSION"; then eval '
+        echo >>config.log "
+funcs: ${FUNCNAME[*]}
+lines: ${BASH_LINENO[*]}"
+    '; fi
     echo $compiler "$@" >> config.log
     $compiler "$@" >> config.log 2>&1 || return $?
     # Test passed. If this is an --enable-werror build, rerun