configure: Add a check for the bash version

Our scripts do not work with older versions of the bash, like the
default Bash 3 from macOS (e.g. we use the "|&" operator which has
been introduced in Bash 4.0). Add a check to make sure that we use
at least version 4 to avoid that the users run into problems later.

Message-Id: <20200925143852.227908-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

 #!/usr/bin/env bash
 
+if [ -z "${BASH_VERSINFO[0]}" ] || [ "${BASH_VERSINFO[0]}" -lt 4 ] ; then
+    echo "Error: Bash version 4 or newer is required for the kvm-unit-tests"
+    exit 1
+fi
+
 srcdir=$(cd "$(dirname "$0")"; pwd)
 prefix=/usr/local
 cc=gcc