ci: Allow the user to force an in-tree cleanup before verification
Introduce the environment option CI_FORCE:
* ci_verify_configure.sh is known to fail if an existing build
configuration is found in the top-level directory.
Setting CI_FORCE=1 will run `make distclean` before verification.
* ci_verify_makefiles.sh cannot be reliably executed if random
object files are found in the top-level directory.
Setting CI_FORCE=1 will run `rm *.o *.obj` before verification.
* ci_verify_cmake.sh is not known at this time to fail for similar
reasons; but if it does, we will use CI_FORCE to trigger any
necessary pre-build cleanup.
4 files changed
