configure: add --with-bash option for cross-compilation

[skirmess]

Problem

AC_PATH_PROG([BASHPATH], [bash]) finds bash on the build host and
substitutes that path into shell scripts via @BASHPATH@. When
cross-compiling, the build host path (e.g. /usr/bin/bash on
Fedora) differs from the target system path (e.g. /bin/bash on
OpenWrt, /usr/local/bin/bash on FreeBSD), resulting in scripts
with a broken shebang on the target.

Solution

Add a --with-bash=PATH configure option that allows explicitly
specifying the bash path on the target system. When not specified,
the existing AC_PATH_PROG auto-detection behaviour is unchanged,
so this is a fully backwards-compatible change.

Cross-compilation packagers can then set the correct target path:

./configure --with-bash=/bin/bash

Testing

# explicit path
./configure --with-bash=/bin/bash   # BASHPATH='/bin/bash'

# auto-detect (unchanged behaviour)
./configure                          # checking for bash... /usr/bin/bash

[knet-jenkins]

Can one of the project admins check and authorise this run please: https://haci.fast.eng.rdu2.dc.redhat.com/job/corosync-qdevice/job/corosync-qdevice-pipeline/job/PR-40/1/input