Arrange to --disable-shared by default for VxWorks

This change makes sure that shared libraries for VxWorks are
only built on explicit request, when configured with --enable-shared.

As the support to build shared libs gets in very incrementally,
this provides us with a robust way to guard the relevant pieces
and reduce the risks of accidentally breaking a platform not yet
ready for it.

2022-09-30  Olivier Hainque  <hainque@adacore.com>

	* configure.ac (*vxworks*): If enable_shared is not
	set, set to "no" and add --disable-shared to target and
	host_configargs.
	* configure: Regenerate.